message: add some accessors to xplane_builder.
diff: @@ -92,6 +92,9 @@ class XEventBuilder : public XStatsBuilder<XEvent> {
  XEventBuilder(const XLine* line, XEvent* event)
  : XStatsBuilder<XEvent>(event), line_(line), event_(event) {}
  + int64 OffsetPs() const { return event_->offset_ps(); }
  + int64 MetadataId() const { return event_->metadata_id(); }
  +
  void SetOffsetPs(int64 offset_ps) { event_->set_offset_ps(offset_ps); }
  void SetOffsetNs(int64 offset_ns) { SetOffsetPs(NanosToPicos(offset_ns)); }
  