The V8.Execute histogram is not free and can cause more overhead
than expected. This CL is guarding slower histograms behind a new
--slow-histograms flag.

For now --slow-histograms is enabled by default. Once all
chrome-side changes and benchmark changes have landed it will be
disabled by default.

--dump-counters will automatically enable --slow-histograms.

The goal is to not report slow histograms on UMA by default on stable:
- 100% reporting on canary/dev/beta
- 1% reporting on stable or specific finch experiments

Chrome-side feature: https://crrev.com/c/3065464

Bug: v8:11946
Change-Id: I23c782288e10ceb76323d72eceea9170739fd543
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3067318
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76041}