Move FrameTree pointer from LocalFrame to Frame

This is a staging patch for adding RemoteFrames to the frame tree. For
now FrameTree still returns LocalFrames, but that will change after all
the FrameTree method callers are converted to deal with Frames.

This also changes the frameClient() method to return a FrameClient
or a FrameLoaderClient depending on the instantiated Frame class.

BUG=346764
R=dcheng@chromium.org

Review URL: https://codereview.chromium.org/300513004

git-svn-id: svn://svn.chromium.org/blink/trunk@174897 bbb929c8-8fbe-4397-9dbb-9b2b20218538