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Fix the DCHECK in ValidateLayout for symmetric channel layouts.

Add 'const' to methods that don't mutate class members.

Include "base/macros.h" for the DISALLOW_COPY_AND_ASSIGN macro.

R=dalecurtis@chromium.org

Review URL: https://codereview.chromium.org/645853011

Cr-Commit-Position: refs/heads/master@{#300612}