cmd/trace: handle Sync event at the beginning of the trace
Currently the code assumes that there's no Sync event at the start of
the trace, but this hasn't been correct for some time. Count Syncs and
look for at least one instead of looking for zero.
Fixes #73962.
Change-Id: I2b4199a21c699c5b50b3d5add37dc46a515108c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/678555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>