Fix a crash on Linux which occurs during drag drop operations in the renderer while dereferncing a NULL TabContentsWrapper pointer.
Fix a crash on Linux which occurs during drag drop operations in the renderer while dereferncing
a NULL TabContentsWrapper pointer.

Fix is to initialize the TCW member in the DragMotion event in the WebDragDestGtk object. This function is called
at the start of the D&D operation.

Added DCHECKs wherever we deref this member. There is no need to DCHECK the bookmark helper pointer
as a TCW always has a bookmark helper object.

BUG=89388
Review URL: http://codereview.chromium.org/7830022

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@99253 0039d316-1c4b-4281-b951-d872f2087c98

ltilve/chromium,chuan9/chromium-crosswalk,krieger-od/nwjs_chromium.src,zcbenz/cefode-chromium,nacl-webkit/chrome_deps,Pluto-tv/chromium-crosswalk,M4sse/chromium.src,hgl888/chromium-crosswalk,markYoungH/chromium.src,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,nacl-webkit/chrome_deps,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,Jonekee/chromium.src,dednal/chromium.src,pozdnyakov/chromium-crosswalk,anirudhSK/chromium,zcbenz/cefode-chromium,anirudhSK/chromium,junmin-zhu/chromium-rivertrail,junmin-zhu/chromium-rivertrail,Pluto-tv/chromium-crosswalk,Jonekee/chromium.src,patrickm/chromium.src,timopulkkinen/BubbleFish,ChromiumWebApps/chromium,hgl888/chromium-crosswalk-efl,robclark/chromium,anirudhSK/chromium,crosswalk-project/chromium-crosswalk-efl,ondra-novak/chromium.src,Pluto-tv/chromium-crosswalk,krieger-od/nwjs_chromium.src,rogerwang/chromium,rogerwang/chromium,markYoungH/chromium.src,hgl888/chromium-crosswalk-efl,Chilledheart/chromium,zcbenz/cefode-chromium,Just-D/chromium-1,anirudhSK/chromium,anirudhSK/chromium,zcbenz/cefode-chromium,hgl888/chromium-crosswalk-efl,nacl-webkit/chrome_deps,M4sse/chromium.src,robclark/chromium,Jonekee/chromium.src,Fireblend/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,ondra-novak/chromium.src,PeterWangIntel/chromium-crosswalk,pozdnyakov/chromium-crosswalk,mogoweb/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,pozdnyakov/chromium-crosswalk,mogoweb/chromium-crosswalk,hgl888/chromium-crosswalk-efl,mohamed--abdel-maksoud/chromium.src,robclark/chromium,robclark/chromium,timopulkkinen/BubbleFish,markYoungH/chromium.src,patrickm/chromium.src,ondra-novak/chromium.src,junmin-zhu/chromium-rivertrail,ChromiumWebApps/chromium,zcbenz/cefode-chromium,junmin-zhu/chromium-rivertrail,mohamed--abdel-maksoud/chromium.src,ltilve/chromium,axinging/chromium-crosswalk,axinging/chromium-crosswalk,dushu1203/chromium.src,Just-D/chromium-1,robclark/chromium,nacl-webkit/chrome_deps,axinging/chromium-crosswalk,bright-sparks/chromium-spacewalk,ChromiumWebApps/chromium,ChromiumWebApps/chromium,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,Just-D/chromium-1,hujiajie/pa-chromium,axinging/chromium-crosswalk,M4sse/chromium.src,mogoweb/chromium-crosswalk,chuan9/chromium-crosswalk,krieger-od/nwjs_chromium.src,markYoungH/chromium.src,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,chuan9/chromium-crosswalk,bright-sparks/chromium-spacewalk,robclark/chromium,robclark/chromium,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,chuan9/chromium-crosswalk,ondra-novak/chromium.src,Just-D/chromium-1,chuan9/chromium-crosswalk,Chilledheart/chromium,timopulkkinen/BubbleFish,patrickm/chromium.src,fujunwei/chromium-crosswalk,rogerwang/chromium,keishi/chromium,ondra-novak/chromium.src,ChromiumWebApps/chromium,littlstar/chromium.src,rogerwang/chromium,ltilve/chromium,dednal/chromium.src,ltilve/chromium,nacl-webkit/chrome_deps,ChromiumWebApps/chromium,krieger-od/nwjs_chromium.src,fujunwei/chromium-crosswalk,krieger-od/nwjs_chromium.src,keishi/chromium,keishi/chromium,keishi/chromium,jaruba/chromium.src,patrickm/chromium.src,Chilledheart/chromium,Chilledheart/chromium,M4sse/chromium.src,dednal/chromium.src,nacl-webkit/chrome_deps,TheTypoMaster/chromium-crosswalk,junmin-zhu/chromium-rivertrail,PeterWangIntel/chromium-crosswalk,chuan9/chromium-crosswalk,ChromiumWebApps/chromium,dushu1203/chromium.src,mogoweb/chromium-crosswalk,keishi/chromium,hujiajie/pa-chromium,Just-D/chromium-1,keishi/chromium,zcbenz/cefode-chromium,mogoweb/chromium-crosswalk,M4sse/chromium.src,littlstar/chromium.src,ChromiumWebApps/chromium,jaruba/chromium.src,mohamed--abdel-maksoud/chromium.src,patrickm/chromium.src,ltilve/chromium,ltilve/chromium,pozdnyakov/chromium-crosswalk,Fireblend/chromium-crosswalk,krieger-od/nwjs_chromium.src,hgl888/chromium-crosswalk-efl,dednal/chromium.src,rogerwang/chromium,krieger-od/nwjs_chromium.src,littlstar/chromium.src,nacl-webkit/chrome_deps,hujiajie/pa-chromium,hgl888/chromium-crosswalk,timopulkkinen/BubbleFish,krieger-od/nwjs_chromium.src,ChromiumWebApps/chromium,littlstar/chromium.src,ChromiumWebApps/chromium,keishi/chromium,Fireblend/chromium-crosswalk,jaruba/chromium.src,dushu1203/chromium.src,dednal/chromium.src,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Jonekee/chromium.src,mohamed--abdel-maksoud/chromium.src,bright-sparks/chromium-spacewalk,markYoungH/chromium.src,keishi/chromium,axinging/chromium-crosswalk,nacl-webkit/chrome_deps,M4sse/chromium.src,patrickm/chromium.src,rogerwang/chromium,rogerwang/chromium,hujiajie/pa-chromium,M4sse/chromium.src,mogoweb/chromium-crosswalk,Jonekee/chromium.src,hujiajie/pa-chromium,ondra-novak/chromium.src,Chilledheart/chromium,markYoungH/chromium.src,ondra-novak/chromium.src,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk-efl,anirudhSK/chromium,markYoungH/chromium.src,zcbenz/cefode-chromium,anirudhSK/chromium,Fireblend/chromium-crosswalk,jaruba/chromium.src,M4sse/chromium.src,zcbenz/cefode-chromium,mohamed--abdel-maksoud/chromium.src,ltilve/chromium,PeterWangIntel/chromium-crosswalk,dushu1203/chromium.src,Fireblend/chromium-crosswalk,pozdnyakov/chromium-crosswalk,dushu1203/chromium.src,Fireblend/chromium-crosswalk,jaruba/chromium.src,ltilve/chromium,hujiajie/pa-chromium,bright-sparks/chromium-spacewalk,hujiajie/pa-chromium,littlstar/chromium.src,krieger-od/nwjs_chromium.src,bright-sparks/chromium-spacewalk,pozdnyakov/chromium-crosswalk,zcbenz/cefode-chromium,junmin-zhu/chromium-rivertrail,anirudhSK/chromium,jaruba/chromium.src,M4sse/chromium.src,crosswalk-project/chromium-crosswalk-efl,fujunwei/chromium-crosswalk,ondra-novak/chromium.src,junmin-zhu/chromium-rivertrail,anirudhSK/chromium,TheTypoMaster/chromium-crosswalk,junmin-zhu/chromium-rivertrail,hgl888/chromium-crosswalk,littlstar/chromium.src,robclark/chromium,littlstar/chromium.src,ltilve/chromium,crosswalk-project/chromium-crosswalk-efl,Pluto-tv/chromium-crosswalk,timopulkkinen/BubbleFish,junmin-zhu/chromium-rivertrail,hgl888/chromium-crosswalk-efl,timopulkkinen/BubbleFish,Chilledheart/chromium,bright-sparks/chromium-spacewalk,ChromiumWebApps/chromium,rogerwang/chromium,krieger-od/nwjs_chromium.src,Jonekee/chromium.src,mohamed--abdel-maksoud/chromium.src,timopulkkinen/BubbleFish,jaruba/chromium.src,bright-sparks/chromium-spacewalk,keishi/chromium,axinging/chromium-crosswalk,littlstar/chromium.src,fujunwei/chromium-crosswalk,zcbenz/cefode-chromium,hgl888/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,rogerwang/chromium,crosswalk-project/chromium-crosswalk-efl,patrickm/chromium.src,nacl-webkit/chrome_deps,mohamed--abdel-maksoud/chromium.src,jaruba/chromium.src,Just-D/chromium-1,patrickm/chromium.src,hgl888/chromium-crosswalk-efl,Jonekee/chromium.src,mogoweb/chromium-crosswalk,crosswalk-project/chromium-crosswalk-efl,anirudhSK/chromium,fujunwei/chromium-crosswalk,nacl-webkit/chrome_deps,Just-D/chromium-1,pozdnyakov/chromium-crosswalk,ChromiumWebApps/chromium,dednal/chromium.src,pozdnyakov/chromium-crosswalk,timopulkkinen/BubbleFish,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,dushu1203/chromium.src,pozdnyakov/chromium-crosswalk,hgl888/chromium-crosswalk-efl,fujunwei/chromium-crosswalk,junmin-zhu/chromium-rivertrail,jaruba/chromium.src,jaruba/chromium.src,hujiajie/pa-chromium,axinging/chromium-crosswalk,anirudhSK/chromium,Jonekee/chromium.src,TheTypoMaster/chromium-crosswalk,zcbenz/cefode-chromium,pozdnyakov/chromium-crosswalk,mogoweb/chromium-crosswalk,dednal/chromium.src,Pluto-tv/chromium-crosswalk,krieger-od/nwjs_chromium.src,Jonekee/chromium.src,TheTypoMaster/chromium-crosswalk,patrickm/chromium.src,hujiajie/pa-chromium,Chilledheart/chromium,rogerwang/chromium,axinging/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ondra-novak/chromium.src,Chilledheart/chromium,dednal/chromium.src,hujiajie/pa-chromium,dushu1203/chromium.src,Jonekee/chromium.src,dednal/chromium.src,timopulkkinen/BubbleFish,dushu1203/chromium.src,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,mohamed--abdel-maksoud/chromium.src,Just-D/chromium-1,nacl-webkit/chrome_deps,keishi/chromium,junmin-zhu/chromium-rivertrail,dushu1203/chromium.src,chuan9/chromium-crosswalk,dednal/chromium.src,bright-sparks/chromium-spacewalk,M4sse/chromium.src,crosswalk-project/chromium-crosswalk-efl,hujiajie/pa-chromium,robclark/chromium,mogoweb/chromium-crosswalk,Jonekee/chromium.src,bright-sparks/chromium-spacewalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,dushu1203/chromium.src,markYoungH/chromium.src,crosswalk-project/chromium-crosswalk-efl,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,mogoweb/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,fujunwei/chromium-crosswalk,M4sse/chromium.src,hgl888/chromium-crosswalk,robclark/chromium,markYoungH/chromium.src,markYoungH/chromium.src,pozdnyakov/chromium-crosswalk,dushu1203/chromium.src,crosswalk-project/chromium-crosswalk-efl,Pluto-tv/chromium-crosswalk,markYoungH/chromium.src,Chilledheart/chromium,dednal/chromium.src,Pluto-tv/chromium-crosswalk,Fireblend/chromium-crosswalk,jaruba/chromium.src,timopulkkinen/BubbleFish,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk-efl,hgl888/chromium-crosswalk,keishi/chromium,anirudhSK/chromium,timopulkkinen/BubbleFish