Oilpan: move ScriptController+WindowProxy to the heap.

Turn LocalFrame's ScriptController into a GCed object, letting it
correctly trace its member reference back to its owner.

Having it be heap allocated also simplifies how its LocalFrame is kept
alive across calls that invoke scripts which indirectly detaches/removes
the frame.

Transitively do the exact same for ScriptController's owned WindowProxies.

R=mkwst,haraken
BUG=340522

Review URL: https://codereview.chromium.org/627933002

git-svn-id: svn://svn.chromium.org/blink/trunk@183255 bbb929c8-8fbe-4397-9dbb-9b2b20218538