Move the Widget hierarchy to the Oilpan heap.

Turn Widget into a GCable object, along with its derived objects
(PluginView, FrameView, Scrollbar, PopupContainer).

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/603193005

git-svn-id: svn://svn.chromium.org/blink/trunk@183583 bbb929c8-8fbe-4397-9dbb-9b2b20218538