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Before: Schedule the computation events using a RepeatingTimer and use a
session number to address potential race conditions.

After: Use a flag |floc_computation_in_progress_| to guard the process
from ComputeFloc to OnComputeFlocCompleted. Incoming computation will be
ignored if there’s one ongoing. After each OnComputeFlocCompleted, a
ComputeFloc event will be scheduled 24 hours later / override the
existing scheduled event.

Why: The new pattern would be more suitable for a future plan where more
computation triggers are introduced (e.g. kHistoryDelete). In that case,
the caller can simply call
ComputeFloc(ComputeFlocTrigger::kHistoryDelete) without needing to reset
the timer.


Bug: 1062736
Change-Id: Ic128262272c71be3bc8998bb192ff7c7b615faa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359394
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799547}