[scheduler] Fix tracing for WebScopedVirtualTimePauser.

- Emit trace event when the time is unpaused when Pauser is destroyed.
- Randomise trace_id to avoid collisions with other events.

R=alexclarke@chromium.org
TBR=haraken@chromium.org

Change-Id: I7492cf1975dc6735dda143b981943b73d2f65b13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601033
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657696}
2 files changed