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It is unused, and so are DeprecatedGetMockTickClock(),
LegacyMockTickClock and LegacyMockClock.

Fixed: 789079
Change-Id: Ic5c504a19dae5217c6e7971cecb3c3325d533c5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2496506
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821556}