cmd/trace: GC appears to start 25 days earlier than trace started, in UI only

[mknyszek]

@prattmic recently noticed in a trace that begin tracing during the GC cycle that the concurrent mark phase appears to start 25 days before the trace started.

This is due to a bug in cmd/trace -- it wasn't updated to deal with a Sync event appearing at the start of the trace.

[gopherbot]

Change https://go.dev/cl/678555 mentions this issue: cmd/trace: handle Sync event at the beginning of the trace