This CL adds timestamps to coverage updates. This is useful for clients,
because between requesting a coverage update and collecting coverage data
significant time may pass. This change allows precise attribution at what
time a coverage update was taken.

Change-Id: I65a9cccb0171e892b7dbe35d105d3ca246ba08d0
Bug: chromium:1022031
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1992435Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65696}