Revert "Remove toV8FastForMainWorld."

This reverts revision 154673.

Reason: We want to see if the dromaeo regression goes away, see
crbug.com/264056

TBR=marja,abarth
BUG=264056

Review URL: https://codereview.chromium.org/20668002

git-svn-id: svn://svn.chromium.org/blink/trunk@154980 bbb929c8-8fbe-4397-9dbb-9b2b20218538