Revert "Completely removed the Extensions3D class"

This reverts r164652, which broke the Android build (I think).

TBR=bajones@chromium.org
BUG=326382

Review URL: https://codereview.chromium.org/126823005

git-svn-id: svn://svn.chromium.org/blink/trunk@164655 bbb929c8-8fbe-4397-9dbb-9b2b20218538