Previously, TestMockTimeTaskRunner had only supported vending out TimeTicks and TickClocks. Now it supports mock Time and mock Clocks too (they start at the Unix epoch).

This CL also shortens the names of the methods that vend the current time and time ticks to Now() and NowTicks(), respectively; removes using the "base" namespace prefix inside the "base" namespace; and provides a clean implementation for FastForwardUntilNoTasksRemain() instead of a hacky solution.

BUG=329911
TBR=phajdan.jr@chromium.org  # OOO.

Review URL: https://codereview.chromium.org/899863002

Cr-Commit-Position: refs/heads/master@{#316219}