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It merely needs a TaskRunner to PostTask() to.

This allows the unittest to merely use a base::Thread and alleviates
issue 646443.

Also did some modern style cleanup:
 - ThreadChecker => SequenceChecker (no state is thread-affine)
 - const scoped_refptr<>& => scoped_refptr<> + std::move
   (avoids a refcount bump when handing off ownership:
    https://groups.google.com/a/chromium.org/d/topic/chromium-dev/TlL1D-Djta0/discussion)

BUG=646443
NO_DEPENDENCY_CHECKS=true

Review-Url: https://codereview.chromium.org/2335193007
Cr-Commit-Position: refs/heads/master@{#418981}