Remove toV8FastForMainWorld from custom bindings.

It is no more. It has ceased to be. (See
https://chromiumcodereview.appspot.com/19799002 ).

BUG=NONE

Review URL: https://chromiumcodereview.appspot.com/23020013

git-svn-id: svn://svn.chromium.org/blink/trunk@156222 bbb929c8-8fbe-4397-9dbb-9b2b20218538