Cleanup: remove toV8FastForMainWorld from custom bindings, too.

This change got lost between patch sets 3 and 4 when re-landing
https://chromiumcodereview.appspot.com/19799002.

BUG=NONE
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/22399002

git-svn-id: svn://svn.chromium.org/blink/trunk@155596 bbb929c8-8fbe-4397-9dbb-9b2b20218538