Oilpan: remove unused support for removing FrameDestructionObservers.

Remove ability to explicitly remove a FrameDestructionObserver from a
LocalFrame; this is taken care of by weak references instead.

Also make FrameDestructionObserver::observeFrame() private; this will ease
later removal once we can assume Oilpan is enabled-by-default for all
observer implementations.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/584403002

git-svn-id: svn://svn.chromium.org/blink/trunk@182370 bbb929c8-8fbe-4397-9dbb-9b2b20218538