This adds Google benchmark for microbenchmarking C++ code as an
optional dependency.

To enable, add the following to the .gclient before syncing
  "custom_vars": {
      "checkout_google_benchmark": True
  }

Change-Id: Id0eab772dd71558906658ef4bb60e31acd665948
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2275964Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68663}