Remove concept of frame IDs from embedding layer.

Unfortunately, we can't get rid of them completely at this time because
history has a dependency on it. Since history is eventually going to
move into the browser, I've added temporary hacks in WebCore::Frame for
history rather than plumbing the routing ID from WebFrameClient all the
way into WebCore.

BUG=314791
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/168763003

git-svn-id: svn://svn.chromium.org/blink/trunk@167637 bbb929c8-8fbe-4397-9dbb-9b2b20218538