In bug 922912, we folded back gkmedias.dll info xul.dll, so in practice, there
is no default configuration left that exercises GKMEDIAS_SHARED_LIBRARY. And
sure enough, it's been broken for months in many different ways.
The gkmedias intermediate library is however kept for webrtc signaling tests.
The patch removes 455 occurrences of FAIL_ON_WARNINGS from moz.build files, and
adds 78 instances of ALLOW_COMPILER_WARNINGS. About half of those 78 are in
code we control and which should be removable with a little effort.
