Note bug number (bug 846137) for assertion annotation.
Bug 855464:  Optimize nsBlockReflowState::ClearFloats better, given that nsBlockFrame::WidthToClearPastFloats is somewhat expensive.  r=dholbert

This fixes a performance issue that showed up in a profile of the b2g
contacts app (though it's not clear what percentage of the time it was
given the difficulty of understanding output of 'perf').
Bug 855455:  Turn on @supports in release builds.  r=heycam