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The synchronous readback path adds complexity through ui and cc
compositors and will ideally be removed.  The screenshots already use a
post task mechanism to save to disk, so making them use the asynchronous
readback path instead is straightforward.

BUG=332167

Review URL: https://codereview.chromium.org/126373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244859 0039d316-1c4b-4281-b951-d872f2087c98