Remove toV8FastForMainWorld.

It was essentially the same as toV8ForMainWorld; if we already know we're in the
main world, we cannot benefit from faster world inferring.

For more information, pls see the discussion in https://codereview.chromium.org/19697004/.

BUG=

Review URL: https://chromiumcodereview.appspot.com/19799002

git-svn-id: svn://svn.chromium.org/blink/trunk@154673 bbb929c8-8fbe-4397-9dbb-9b2b20218538