Got WebGLRenderingContextBase to work with the "implements" syntax

BUG=348033
R=arv@chromium.org, ch.dumez@samsung.com

Review URL: https://codereview.chromium.org/205243013

git-svn-id: svn://svn.chromium.org/blink/trunk@170000 bbb929c8-8fbe-4397-9dbb-9b2b20218538