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TaskQueueImpl supported registering a callback that is invoked when
a task becomes ready. This was only used by the obsolete agent
interference histogram. Therefore, this CL removes support for this
callback.

Bug: 1019856, 984057
Change-Id: I838b88e13174eff5347de55646d22565b770339c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2297719
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788697}