- Implement exchangeable timer clock for RuntimeCallStats for testing
- Rewrite RuntimeCAllStatsTest to overwrite the default RCS timer

This gets rid of the previous flakiness for these tests due to using
the real platform timer.

Bug: v8:5677
Change-Id: Iff312c7f79ab97407ba1c0c2c72fb0b35a5efdf1
Reviewed-on: https://chromium-review.googlesource.com/760416Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49285}