Completely removed the Extensions3D class

This WebKit-era abstraction serves no useful purpose in the context of blink.

BUG=326382

Review URL: https://codereview.chromium.org/126843002

git-svn-id: svn://svn.chromium.org/blink/trunk@164652 bbb929c8-8fbe-4397-9dbb-9b2b20218538