[PaintHolding] Add UMA to detect prevalence of early input

There is concern about input arriving and being processed before the
user sees anything. Tests indicate that this is possible in the test
environment, but how prevalent is it in the wild. Let's find out.

Bug: 958424
Change-Id: I6e2d896bd9963bc6b7d51cf275a111a580399261
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639664
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Steven Holte <[email protected]>
Commit-Queue: Stephen Chenney <[email protected]>
Auto-Submit: Stephen Chenney <[email protected]>
Cr-Commit-Position: refs/heads/master@{#666701}
5 files changed