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This change adds a metric to measure how often a navigation occurs
where the target URL matches a RenderProcessHost that was destroyed in
the past few seconds.

The new metric, "SiteIsolation.ReusePendingOrCommittedSite.
TimeSinceReusableProcessDestroyed", emits on every navigation. It
records the time (in ms) since a RenderProcessHost with a matching
process lock was destroyed, up to 7 seconds. If no matching host was
destroyed within the past 7 seconds, it records a sentinel value of 10
seconds.

Knowing the most common times since reusable processes were destroyed
will inform a broader experiment to potentially delay subframe process
shutdown by a few seconds to reduce process churn. Including the
sentinel value adds information around how frequently reusable
processes are destroyed, i.e., how much opportunity for process reuse
exists.

Test plan (see "Metrics" section for details on this metric):
https://docs.google.com/document/d/11XruVZzYLSkt4se6IV2O4jFOkKs-QbkRB5NMf4mH_hE

Bug: 894253
Change-Id: Ic15089d1e3b086480942b6afe32525b3c6461895
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575945Reviewed-by: Oliver Li <olivierli@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#846167}