Record resource scheduler UMA

Record UMA on how many requests are in-flight. UMA is recorded only when
the set of in-flight requests changes.

Also, record UMA on highest number of delayable requests in-flight when
a delayable or layout blocking request is in flight. This UMA is
recorded per-request (on select requests).

BUG=720783

Review-Url: https://codereview.chromium.org/2873223002
Cr-Commit-Position: refs/heads/master@{#477447}
3 files changed