Add support for windows def generation from sigs.
Add support for windows def generation from sigs.

Used to generate the export table for the MSVC build of
ffmpeg.  Necessary since we don't want to decorate all the
ffmpeg methods we use with __declspec(dllexport) a la
MEDIA_EXPORT.

Doing things this way allows us to keep the security of
having an explicit whitelist for ffmpeg symbols.

BUG=39887
TEST=MSVC version of ffmpeg builds and runs.  Unittests.

Review URL: https://codereview.chromium.org/11027054

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@160524 0039d316-1c4b-4281-b951-d872f2087c98

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