Move PlatformEventDispatcher implementations to the Oilpan heap.

Make PlatformEventDispatcher a GC mixin, maintaining weak references
to its registered controllers.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/619383002

git-svn-id: svn://svn.chromium.org/blink/trunk@183211 bbb929c8-8fbe-4397-9dbb-9b2b20218538