Add security checks to DOMDataStore::getWrapperForMainWorld.

getWrapper and getWrapperFast already have this check; getWrapperForMainWorld
should have it too.

BUG=
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/22279005

git-svn-id: svn://svn.chromium.org/blink/trunk@155678 bbb929c8-8fbe-4397-9dbb-9b2b20218538