Bug 1198334 (part 1) - Replace the opt-in FAIL_ON_WARNINGS with the opt-out ALLOW_COMPILER_WARNINGS. r=glandium.

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.

--HG--
extra : rebase_source : 82e3387abfbd5f1471e953961d301d3d97ed2973