Math tag is not exposed in Accessibility.

This CL will return MathRole from blink to chromium for math tag.

BUG=429476

Review URL: https://codereview.chromium.org/722653002

git-svn-id: svn://svn.chromium.org/blink/trunk@185264 bbb929c8-8fbe-4397-9dbb-9b2b20218538