Bug 1256535 - Disable C4577 to unblock compilation on VS2015; r=poiru

The previous disabling of this warning on just Key.cpp was not
sufficient because another file from the unified sources list appears
to include the header exhibiting the warning.

MozReview-Commit-ID: rR2XXigTJU

--HG--
extra : rebase_source : b34b42fd729163775cdb2e4c50bb0e6099824112

dom/indexedDB/moz.build

@@ -100,7 +100,7 @@ if CONFIG['GNU_CC']:
 
 if CONFIG['_MSC_VER']:
     # This is intended as a temporary hack to support building with VS2015.
-    SOURCES['Key.cpp'].flags += ['-wd4577']
+    CXXFLAGS += ['-wd4577']
 
 LOCAL_INCLUDES += [
     '/db/sqlite3/src',