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Rather than going through the WindowManager and Display to get
DisplayMetrics, use the instance hanging off the context. This saves
several extra IPCs and reduces time in these functions during startup by
~20ms.

NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/16093031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@204079 0039d316-1c4b-4281-b951-d872f2087c98