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SharedMemoryDataConsumerHandle::Context posts tasks while its lock_
locked. That means a lock held by the TaskRunner will be locked
after |lock_| is locked. On the other hand, the Context may be
destructed in the middle of TaskRunner destruction, which means we
cannot lock |lock_| in the Context destruction. Otherwise, a
lock-order-inversion problem will be introduced.

This CL stops locking |lock_| in the destructor. It is safe because
no one accesses to Content's member variables without a valid
scoped_refptr to the Context.

BUG=651747

Review-Url: https://codereview.chromium.org/2391513002
Cr-Commit-Position: refs/heads/master@{#422614}