Fix drag-hover tab-selection with files dragged from the download manager.
Fix drag-hover tab-selection with files dragged from the download manager.

The issue was that |download_util::DragDownload()| blocked until the drag was done, preventing the message loop from running tasks scheduled by hover_tab_selector.cc, since by default the message loop doesn't allow nested execution.

The fix is to temporarily enable nested tasks on the message loop around the |DragDownload()| call. (This is already done elsewhere for other drags.)

BUG=82608
TEST=manual

Review URL: http://codereview.chromium.org/7153002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@89602 0039d316-1c4b-4281-b951-d872f2087c98

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