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This patch moves some of the predecode image tracking to ImageManager.
The other part depends on crbug.com/647402. Once the TileTaskRunner
can keep a ref on scheduled tiles, then we can remove
TileManager::locked_image_tasks_ as well.

R=enne
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2345833002
Cr-Commit-Position: refs/heads/master@{#419539}