Use composition for LocalDOMWindow's FrameDestructionObserver.

Using inheritance is problematic here since the DOMWindow superclass
needs a virtual Frame* accessor for binding security checks. Currently,
it's believed that FrameDestructionObserver will only be needed for
LocalFrames, so rather than trying to adapt it to handle generic Frames,
this patch hides the FrameDestructionObserver inside the class
implementation with composition.

BUG=425623

Review URL: https://codereview.chromium.org/667713008

git-svn-id: svn://svn.chromium.org/blink/trunk@184578 bbb929c8-8fbe-4397-9dbb-9b2b20218538