Turn on ENABLE_MATHML for Chromium

​https://bugs.webkit.org/show_bug.cgi?id=96960



Reviewed by Adam Barth.



Source/WebCore:



Add the rendering/mathml directory to search.



WebCore.gyp/WebCore.gyp:

Source/WebKit/chromium:



features.gypi:

LayoutTests:



We're enabling MathML now partly to let the automated fuzzers go to work. Some layout tuning will also be done in the

next few days, so we expect MathML pixel tests to be changing during that time.

