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This change adds a base::Clock pointer to the base TaskQueue tasks and
updates all time related calls in tasks and tests in:

components/offline_pages/core/prefetch/tasks/

Bug: 906903
Change-Id: Ia701af7d49dcd5376c903598e61f490721149df2
Reviewed-on: https://chromium-review.googlesource.com/c/1344829Reviewed-by: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Dan H <harringtond@google.com>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611010}