Prevent stacking of HangWatcher trace events.

Previously trace events of prolonged hangs kept being emitted
but always relative to the start of the original deadline.

This makes each sucessive events start at the beginning of the last
monitoring period which is less busy visually in traces and also more
representative since the hang began within the monitoring period and
not after it.

Change-Id: Ica7b7fe3da6033ce875ebb9d5ae904139c7c4af5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6227266
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Commit-Queue: Olivier Li <olivierli@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1416181}