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Although passing SkBitmap by value doesn't copy the underlying pixels, it indeed
involves memcpy (~64 bytes to be precise) to replicate the settings from the
other bitmap in the copy constructor.

This overhead, albeit minor, can be avoided by passing SkBitmap as reference.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/597553002

Cr-Commit-Position: refs/heads/master@{#296330}