Added support for IMPLEMENTATION_COLOR_READ_FORMAT and _TYPE to WebGL.

Will be tested by the WebGL conformance suite on the GPU bots.

Rebaselined affected layout tests. Examined the related code
and do not believe any code changes are needed in the
Inspector; the new queries are not parts of the OpenGL state.

BUG=363842

Review URL: https://codereview.chromium.org/239113011

git-svn-id: svn://svn.chromium.org/blink/trunk@171916 bbb929c8-8fbe-4397-9dbb-9b2b20218538