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This CL takes Task out of the reference loop, so that TaskSource
refers to its TaskRunner directly. Task can be replaced by PendingTask
as a result (will be done as a follow-up).
A TaskRunner reference is cleared when the TaskSource becomes empty
and set by the caller when pushing a task to a Sequence.


Change-Id: I9ffeca51d392ba286494ef85bac9697da48b893d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506769
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643019}