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This CL turns const by value return values into const by ref.

Replacing const value by a const ref when a member variable is safe, because
the member variable will live at least as long as the temporary object returned
now. It can also be more efficient, because returning a ref can spare copying.

BUG=393155

Review URL: https://codereview.chromium.org/646853003

Cr-Commit-Position: refs/heads/master@{#299161}