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Follow-up CL to address comments on https://codereview.chromium.org/787903003
after the CL landed:
- cleanup #includes in image_fetcher.{h,mm}
- use const ref to scoped_refptr to avoid inc/dec ref churn
- prefer base::ThreadTaskRunnerHandle::Get() to MessageLoopProxy::current()
- remove std::string concatenation

BUG=None

Review URL: https://codereview.chromium.org/805713004

Cr-Commit-Position: refs/heads/master@{#308347}