Remove toV8Fast.

The performance-critical cases should already use
setV8ReturnValue(|Fast|ForMainWorld).

This is a follow-up change to https://codereview.chromium.org/19566002/ , to
clean up the temporary mess of having many conversion functions.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/23083004

git-svn-id: svn://svn.chromium.org/blink/trunk@156232 bbb929c8-8fbe-4397-9dbb-9b2b20218538