Measure time from requesting GC on background thread until the main
thread starts the collection. Also add trace event for the GC on the
background thread.

Bug: v8:10315
Change-Id: I765ce0df8a1c31e4f64661ea8dc0b6a4685a3132
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2489681Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70707}