Handle subtree layout roots.

FTA previously assumed that the RenderView would be the first block to beginLayout, but FrameView::layout may begin at a different block in some cases.

With this change FTA correctly detects the start and end of layout in such a case, and treats the subtree layout root as an independent cluster.

BUG=332253

Review URL: https://codereview.chromium.org/165583002

git-svn-id: svn://svn.chromium.org/blink/trunk@167225 bbb929c8-8fbe-4397-9dbb-9b2b20218538