Bug 1124033 - Disable C4312 in dom/plugins/ipc; r=ehsan

This was in a previously reviewed patch. I dropped the change because I
thought the underlying warning had been fixed. I was wrong.

MozReview-Commit-ID: J9B34YhJ3z0

--HG--
extra : source : ea446df75d17331a7c0736ef3303bf9a07d8d9f0

dom/plugins/ipc/moz.build

@@ -152,3 +152,6 @@ if CONFIG['_MSC_VER']:
     # This is intended as a temporary hack to support building with VS2015.
     # conversion from 'X' to 'Y' requires a narrowing conversion
     CXXFLAGS += ['-wd4838']
+
+    # 'type cast': conversion from 'unsigned int' to 'HIMC' of greater size
+    CXXFLAGS += ['-wd4312']