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As described in bug 396413, it was possible for a posted task to be run
after CastReceiverImpl/FrameReceiver were destroyed.  This change fixes
the bug by having FrameReceiver post a cancelable task (i.e., using its
WeakPtr) instead.

BUG=396413

Review URL: https://codereview.chromium.org/491953003

Cr-Commit-Position: refs/heads/master@{#292244}