Implement setRemoteWebLayer for WebRemoteFrame.

Needed so OOPI can start trying to render pages using RemoteFrames. For
now, WebLocalFrame still needs to keep an implementation, but once the
RemoteFrame infrastructure is up and running, this can be completely
moved into RemoteFrame.

Also lift the logic for registering and unregistering the graphics layer
and managing compositing updates into Frame as well, so the web
implementation becomes just a simple forwarding call.

BUG=346764
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/364143002

git-svn-id: svn://svn.chromium.org/blink/trunk@177492 bbb929c8-8fbe-4397-9dbb-9b2b20218538