[UKM] Switch Blink.UpdateTime to sampling based on frame count

In an obvious-after-the-fact manner, sampling rendering core
update times based on when a frame passes a time interval leads to
over-sampling longer frames (because they are more likely to be the
frame that crosses the next sampling threshold).

Switch to a frame count based interval between samples to avoid
this problem. Set the mean frames between samples to be about 30s
worth at 60fps. This also removes the less than ideal repeated
counting of very long frames and is overall much simpler and more
efficient to implement (fewer time queries).

Update the test too.

R: rkaplow@chromium.org, vmpstr@chromium.org
Bug: 953839
Change-Id: I6eb6238b8d23cc6cd91cd963b4ed39e913627bf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572193
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653907}
4 files changed