Expose WEBGL_debug_renderer_info through a chromium finch experiment.

This is the blink side part: use a FrameLoaderClient query to decide
whether to expose this extension or not.  The decidion making will be
in the chromium side.

BUG=309831
TEST=no behavioral change until chromium side lands

Review URL: https://codereview.chromium.org/51323003

git-svn-id: svn://svn.chromium.org/blink/trunk@161008 bbb929c8-8fbe-4397-9dbb-9b2b20218538