ui/gfx: Make use of SkAutoLockPixels to automatically lock and unlock bitmap pixels properly.
ui/gfx: Make use of SkAutoLockPixels to automatically lock and unlock bitmap pixels properly.

R=asvitkine@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9858001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@128921 0039d316-1c4b-4281-b951-d872f2087c98

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