Bug 867530 - Move PresArena poisoning code to MFBT.  r=roc

--HG--
rename : layout/base/tests/cpp-tests/TestPoisonArea.cpp => mfbt/tests/TestPoisonArea.cpp
Bug 867530 - Initialize the poison value in XPCOM startup and annotate the values in crash reporter.  r=bsmedberg
Bug 867530 - Make the current poison consumers use the new API.  r=roc
Bug 868906 - Make GetChildFrameContainingOffset() stable.  r=roc
Bug 869529 - Fix LiveRangeAllocator loopWorkList handling. r=bhackett
Bug 867121 - Snapped view: Favicons too close to text in awesome screen.r=mbrubeck