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This is an optimization to speed up in-page navigations. We want those to be
blazingly fast, and currently the pixel readback done while taking a
screenshot takes 40ms on a GPU on Pixel. This may be reverted once
screenshotting is optimizaed.

Also changed the background color shown bu GestureNav when there is no
screenshot available from gray to white to make it look less broken.

BUG=379983

Review URL: https://codereview.chromium.org/417343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285797 0039d316-1c4b-4281-b951-d872f2087c98