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The new benchmark takes all the metrics from the v8.browsing benchmarks and
provides an additional --enable-rcs to switch on the more expensive runtime
call stats metrics.

- Introduce new _LoadingBaseClusterTelemetry base class
- Introduce AugmentOptionsForV8BrowsingMetrics and V8BrowsingShouldAddValue
  helpers to share metrics setup code with v8.browsing benchmarks
- Add new V8LoadingClusterTelemetry benchmark

Bug: chromium:883322
Change-Id: I67287fb046993a4cb687ac57073f81a7c006d361
Reviewed-on: https://chromium-review.googlesource.com/1224448
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592134}