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VideoResourceUpdater uses weak pointers within the class only.
Hence, it does not have to inherit SupportsWeakPtr.

BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2838713004
Cr-Commit-Position: refs/heads/master@{#467883}