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This patch refactors some tile manager code into ImageManager with
an intent of moving all image related stuff (pre-decode images /
checker images) into that class.

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

Review-Url: https://codereview.chromium.org/2339703005
Cr-Commit-Position: refs/heads/master@{#418963}