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This change adds use counters for counting V8 function calls in detached
windows. They aren't emitted the usual way, because the use counter
infrastructure doesn't work on detached windows. Instead they're plumbed
through instance counters to be emitted via UKM.

NOTE! This re-introduces the following change (but without web feature
definitions, which were redundant):
https://chromium-review.googlesource.com/c/chromium/src/+/1925839

TBR=rkaplow@chromium.org,asvitkine@chromium.org
rkaplow or asvitkine: Please approve changes to ukm.xml and
process_memory_metrics_emitter.cc; these files haven't changed
since 1925839, which Robert approved before.

Bug: chromium:1018156
Change-Id: If16c3a85bfe539c202561d435576f1df80f060bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1958487
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723333}