Add missing forward declare for WebFrame.

I removed a couple forward declares that were no longer needed in Blink,
and it caused the chrome build to fail.

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/190953010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256045 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256045