Add  Blink GC, and global renderer GC sums

Adds the following UMA metricss:
- BlinkGC.TimeForGCCycle: Time spent in Blink GC for a complete cycle. This
  includes marking, the atomic pause, and sweeping.
- BlinkGC.TimeForNestedInV8: Time spent in Blink GC events that are nested
  in V8 GC events.

Bug: 843903
Change-Id: Ide371e4d599b6c0f19593d9d6c77ed88a86704c5
Reviewed-on: https://chromium-review.googlesource.com/c/1283369
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601060}
6 files changed