*Scan: Update histograms.xml to cover mutator events

UMA hists currently only display events performed by the Scanner thread.
This CL fixes it. In addition, it adds the stack scanning event.

Bug: 11297512
Change-Id: I7b970fd6aa7d93fc878b7c8dbced63f37a9f5002
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2988316


Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#898411}