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ScopedTaskEnvironment is sufficient for the purposes of power/ml
unit tests, so remove usages of TestMockTimeTaskRunner. This also
removes need to support both TestMockTimeTaskRunner *and*
ScopedTaskEnvironment in FakeBookClock.

Bug: 917580
Test: All related unit tests still pass.

Change-Id: Iec413bf52422a12462f8ff99ef5e02e23907925a
Reviewed-on: https://chromium-review.googlesource.com/c/1393896Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Prashant Malani <pmalani@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619877}