Updated ratified extensions to be advertised without the WEBKIT_ prefix

(Take 2. Previous patch here: https://codereview.chromium.org/145093003/)
Extensions can still be queried with WEBKIT_, but getSupportedExtensions
will return the unprefixed version.

TBR=kbr@chromium.org
BUG=340531

Review URL: https://codereview.chromium.org/145273029

git-svn-id: svn://svn.chromium.org/blink/trunk@167121 bbb929c8-8fbe-4397-9dbb-9b2b20218538