Updated ratified extensions to be advertised without the WEBKIT_ prefix

Extensions can still be queried with WEBKIT_, but getSupportedExtensions
will return the unprefixed version.

BUG=340531

Review URL: https://codereview.chromium.org/145093003

git-svn-id: svn://svn.chromium.org/blink/trunk@166477 bbb929c8-8fbe-4397-9dbb-9b2b20218538