Revert of Updated ratified extensions to be advertised without the WEBKIT_ prefix (https://codereview.chromium.org/145093003/)

Reason for revert:
Broke WebGL Conformance test conformance_extensions_get_extension

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166477

TBR=kbr@chromium.org,zmo@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=340531

Review URL: https://codereview.chromium.org/140433010

git-svn-id: svn://svn.chromium.org/blink/trunk@166526 bbb929c8-8fbe-4397-9dbb-9b2b20218538