Add main thread event dispatch timestamps to cc::EventMetrics

To be able to report main thread event dispatch breakdowns for
EventLatency metrics, this CL adds relevant timestamps to
cc::EventMetrics. The actual breakdowns will be added to UMAs and UKMs
in follow-up CLs.

Bug: 1054009
Change-Id: Ia78d2614d4a3e2e4e5de75e2305e16888acbeeb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568936
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835193}