heap: Add UMA for marking throughput on main thread

Bug: 945806
Change-Id: I430110195b07519495070e1141b4c431bbcb8073
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547970
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647197}
3 files changed