Some fixes that were required to make the metric recording framework run
better:
  - Set the foreground task runner later so it can still be modified in
    test cases
  - Add Start and Stop methods to TimedScope for more control
  - Clear map of contexts explicitly to avoid it being triggered at the
    end of the destructor when counters are already destroyed and a
    SEGFAULT may occur due to histogram updates during destruction of
    the weak persistent handles.

R=rmcilroy@chromium.org

Bug: chromium:1101749
Change-Id: Ib41c7aeb1aac96f0fa102f0fceadbf7ec2dd78dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2351668Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Emanuel Ziegler <ecmziegler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69422}