Completely removed the Extensions3D class (Take 2)

This WebKit-era abstraction serves no useful purpose in the context of blink.
Previous CL failed due to lingering Android dependencies.

BUG=326382
TBR=kbr@chromium.org, jamesr@chromium.org

Review URL: https://codereview.chromium.org/127163003

git-svn-id: svn://svn.chromium.org/blink/trunk@164714 bbb929c8-8fbe-4397-9dbb-9b2b20218538