Split WebLocalFrame into a distinct subclass of WebFrame.

This also moves a few static methods that don't make sense on WebFrame
to WebLocalFrame.

BUG=346764
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/232133004

git-svn-id: svn://svn.chromium.org/blink/trunk@171396 bbb929c8-8fbe-4397-9dbb-9b2b20218538