Make WindowReachedScreen support async readback

This is the final caller that doesn't fall back or is Aura-only, and so
Aura's snapshot can switch over to only being asynchronous.

BUG=332167

Review URL: https://codereview.chromium.org/127103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245354 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245354