worker: Fix a bad static-cast.
worker: Fix a bad static-cast.

For CompositorWorker, the WorkerGlobalScope is a CompositorWorkerGlobalScope, and not
a DedicatedWorkerGlobalScope. So fix the bad cast by teaching the task about the
WorkerObjectProxy on creation (since WorkerMessagagingProxy already knows about it
without having to depend on the WorkerGlobalScope or the WorkerThread).

BUG=471785

Review URL: https://codereview.chromium.org/1047953002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192810 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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