Move the frame tree into the embedder.

Frame ownership was previously transferred into the embedder layer. As
a result, we can also get rid of the RefPtr<>'s that were previously
used in the frame tree structure to keep frames alive, as the embedder
will only release its reference to the frame when it is removed from
the frame tree.

Several utility methods remain behind in FrameTree since they make more
sense there, but the actual tree structure has been lifted up into
WebFrameImpl.

BUG=340002
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/156123004

git-svn-id: svn://svn.chromium.org/blink/trunk@167231 bbb929c8-8fbe-4397-9dbb-9b2b20218538