Add Blink APIs for frame tree mirroring.

Traditionally, frame tree mutation has been driven from inside the renderer
itself. However, for out-of-process iframes, the embedder needs to mirror the
same frame tree structure between several different renderers.

BUG=346764

Review URL: https://codereview.chromium.org/334483002

git-svn-id: svn://svn.chromium.org/blink/trunk@176159 bbb929c8-8fbe-4397-9dbb-9b2b20218538