Only record EventEnqueuedToMainThread tracepoint if available

This tracepoint records the timestamp when an input event was
queued if the event 'will potentially be considered part of a "web
interaction"' (from code comment). In case no such event has been
queued the timestamp is defaulted to 0. This gives wrong timeline in
tracing viewers, both perfetto and chrome://tracing, since such trace
points will be identified as the start of the tracing. Tracing
timestamps are otherwise related to system start on Linux and we get a
long idle time between 0 and the first correct trace point, making it
hard to navigate the viewer.

The solution is simply to only record this event if the time has
been set.

Bug: 405035569
Change-Id: I5a3d66f917f4ad10cdede0c0d3faafe65766f88c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6376084
Reviewed-by: Michal Mocny <mmocny@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: David Landell <david.landell@xperi.com>
Cr-Commit-Position: refs/heads/main@{#1435890}