This CL adds a performace counter similar to
https://www.chromestatus.com/metrics/feature/timeline/popularity/2238
to estimate how often speculation is disabled in the wild.

Bug: v8:7216, v8:7127
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I421637b386534da4a3aed549a9665870e3b97eb1
Reviewed-on: https://chromium-review.googlesource.com/827012Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Hablich <hablich@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50112}