Update WebFrameClient::didClearWindowObject signature.

The callback is called both for the main and isolated worlds, the new
API indicates in which world the window object was cleared

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/133123008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244485 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244485