The new regexp_backtracks / V8.RegExpBacktracks counter tracks the
number of backtracks performed per regexp execution. The results can
help us prioritize related work.

Note that we only count backtracks in the interpreter. Jitted code is
ignored. With our current regexp tiering strategy, that means we count
backtracks only in the first execution of each regexp.

Chromium CL: https://crrev.com/c/1871601

Bug: v8:9892
Change-Id: I19146d6e9bc2355f210a050e93dd7e856ab066f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1873696
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64467}