Oilpan: reliably track listening PlatformEventControllers.

Turn PlatformEventController into a GC mixin with Oilpan enabled, and
have the PlatformEventDispatcher instance track those via a Persistent
reference.

It seems precarious to rely on the controller lifetimes be maintained by
way of other references 'long enough' for the embedder's platform event
handling layer.

R=mlamouri,mkwst,haraken,timvolodine
BUG=419515

Review URL: https://codereview.chromium.org/618673002

git-svn-id: svn://svn.chromium.org/blink/trunk@183158 bbb929c8-8fbe-4397-9dbb-9b2b20218538