Attach a WebLayer to a frame element for out-of-process iframes.

This patch allows a WebLayer to be attached to an
HTMLFrameOwnerElement and causes it to be composited as
the content of its frame. The WebLayer will contain
compositor frame information received from another process
that has rendered the content of an out-of-process iframe.

BUG=325803

Review URL: https://codereview.chromium.org/104433003

git-svn-id: svn://svn.chromium.org/blink/trunk@163890 bbb929c8-8fbe-4397-9dbb-9b2b20218538