Bug 745131 - Improve how <semantics> determine the visible child. r=karlt

--HG--
rename : layout/mathml/nsMathMLmactionFrame.cpp => layout/mathml/nsMathMLSelectedFrame.cpp
rename : layout/mathml/nsMathMLmactionFrame.h => layout/mathml/nsMathMLSelectedFrame.h
Bug 745131 - Reftests for <semantics>. r=karlt
Bug 814379 - Don't use cached userAgent for fallback when no site-specific UA exists. r=dao