Bug 1256482 - Disable C4312 to unblock compilation on VS2015; r=jesup

As part of unblocking building with VS2015u1 in automation, I'm mass
disabling compiler warnings that are turned into errors. This is not
the preferred mechanism to fix compilation warnings, of course.

Given that the warning is in WebRTC which is third party code, there
isn't much we can do about the warning. However, Google is building
Chrome with Visual Studio 2015, so I wouldn't be surprised if this
were fixed upstream or will be fixed upstream. Then again, we allow
warnings when building WebRTC. So perhaps not.

MozReview-Commit-ID: G6JP9fkCzfn

--HG--
extra : rebase_source : bf5c9a049230bb8e483f6a85bdbc2ca20eb3ab45

dom/media/webrtc/moz.build

@@ -92,5 +92,7 @@ FINAL_LIBRARY = 'xul'
 if CONFIG['_MSC_VER']:
     CXXFLAGS += [
         '-wd4275', # non dll-interface class used as base for dll-interface class
+        '-wd4312', # This is intended as a temporary hack to support building with VS2015
+                   # 'reinterpret_cast': conversion from 'DWORD' to 'HANDLE' of greater size
     ]
     DEFINES['__PRETTY_FUNCTION__'] = '__FUNCSIG__'