Enable PaintMixin for paint-timing
Paint timing now exposes paintTime/presentationTime in addition to
startTime, behind a PaintTimingMixin flag.
See I2P: https://groups.google.com/a/chromium.org/g/blink-dev/c/qNlXvvb3i6g/m/fDBXXZM4AAAJ
Bug: 378827535
Change-Id: I688f78c797df41d9b23d33c6a4ff0bed35cc2103
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6017948
Reviewed-by: Michal Mocny mmocny@chromium.org
Commit-Queue: Noam Rosenthal nrosenthal@chromium.org
Cr-Commit-Position: refs/heads/main@{#1385703}