Reocord UMA counters for the following phases:
- Scavenging roots
- Scavenging object graph in parallel

Bug: chromium:850508
Change-Id: I07210271179ce6801ea6daf3b486b4ce237a3261
Reviewed-on: https://chromium-review.googlesource.com/1199302
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55619}