Removed privledged and debug webgl extension handling.

These extensions should be on all the time.

Dependent on https://codereview.chromium.org/292983005/

BUG=375383

Review URL: https://codereview.chromium.org/296053004

git-svn-id: svn://svn.chromium.org/blink/trunk@174903 bbb929c8-8fbe-4397-9dbb-9b2b20218538