Reland "Remove time overrides from TestingPlatformSupportWithMockScheduler"

This is a reland of f75182ab9e221bf8fc4346b30ce723de2a010dcf

TBR=jbroman@chromium.org, dcheng@chromium.org

Original change's description:
> Remove time overrides from TestingPlatformSupportWithMockScheduler
>
> TestingPlatformSupportWithMockScheduler used time overrides to control time in
> tests. Time overrides are removed as part of the OnionSoup effort. Tests should
> use the mock clock from the TestTaskRunner provided by the platform.
>
> Bug: 919383
> Change-Id: Idf3d44ed2aeff8c73de09671b50b779419146d3b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1591634
> Reviewed-by: Jeremy Roman <jbroman@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Sergio Villar <svillar@igalia.com>
> Cr-Commit-Position: refs/heads/master@{#665724}

Bug: 919383
Change-Id: I76d14136b8f239b90dde32442af3cca0c4efe044
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642554
Reviewed-by: Sergio Villar <svillar@igalia.com>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#665888}