Preparatory patch for moving frame tree into embedder.

This patch paves the way for shifting the responsibility of maintaining
the frame tree to the embedder. WebFrame exposes two new methods to
manipulate the frame tree that are stubbed out for now. A followup
patch in Chrome will update WebFrameClient implementations to use these
new methods, and then the frame tree implementation in Blink will be
moved up into Source/web.

This patch also removes some unused methods from FrameTree, as well as
making some test failures a bit easier to diagnose.

BUG=340002
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/152043002

git-svn-id: svn://svn.chromium.org/blink/trunk@166452 bbb929c8-8fbe-4397-9dbb-9b2b20218538