PCScan: Introduce UMA for PCScan phases

This CL brings in UMA hists for PCScan with the following naming scheme:
PA.PCScan.<ProcessType>.Scanner.<Phase>. It also updates naming scheme
for chromeperf metrics, which now looks as follows:
  pa:pcscan:<process_type>:<scanner|mutator>:<phase>
The new <scanner|mutator> thing is needed to distinguish between scanning
that happens in the scanner thread and scanning that is called from
mutator safepoints (upcoming feature).

Bug: 11297512
Change-Id: I0b1bdb0d7f074816931ebbc1975709e1d7e84feb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2673403


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859737}