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Since we now keep the canvas around in the ContentLayerUpdater, we often
end up getting small invalidations that paint on a large canvas. In those
cases, instead of clearing the entire canvas to transparent, clear only the
portion that will be repainted.

BUG=313494

Review URL: https://codereview.chromium.org/61403003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233539 0039d316-1c4b-4281-b951-d872f2087c98