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Instead of assuming every released resource could be potentially
reusable, manage a list of busy resources and a list of immediately
reusable resources. A busy resource is one which can not be locked for
write.
    
Recheck busy resources before the tile manager is to schedule new tasks,
in AssignGpuMemoryToTiles().
    
If this operation becomes too expensive, the CheckBusyResources()
function should only be called if some resource(s) are returned from the
parent compositor or when ResourcePool releases some resource.

BUG=

Review URL: https://codereview.chromium.org/43753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233374 0039d316-1c4b-4281-b951-d872f2087c98