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=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154673

R=abarth@chromium.org, haraken@chromium.org

Review URL: https://codereview.chromium.org/19799002

git-svn-id: svn://svn.chromium.org/blink/trunk@154988 bbb929c8-8fbe-4397-9dbb-9b2b20218538