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Unlike when this method was originally written, there can now be a
second child, a readback layer.  If the root layer happens to change
while a readback is active, it will never complete, hanging the
rendering.

NOTRY=true
BUG=386337

Review URL: https://codereview.chromium.org/407433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284164 0039d316-1c4b-4281-b951-d872f2087c98