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This patch changes |task_runner_| member in tile manager to be a raw
pointer instead of a scoped_refptr. All the codepaths seem to guarantee
that the task runner outlives tile manager, so there is no need to
create a scoped_refptr a raw pointer at tile manager creation time.

R=ericrk, reveman
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2026663005
Cr-Commit-Position: refs/heads/master@{#397241}