Fix timeline time origin / extent printing with an empty timeline

runtime/vm/timeline.cc

Index: runtime/vm/timeline.cc
diff --git a/runtime/vm/timeline.cc b/runtime/vm/timeline.cc
index 97fdd61772f72cec99feb1a3ee374bf7f8f1c1a1..0ae274ca4c5ccf42540b02cdd841c7493f8f0d18 100644
--- a/runtime/vm/timeline.cc
+++ b/runtime/vm/timeline.cc
@@ -1121,11 +1121,17 @@ void TimelineEventRecorder::ReportTime(int64_t micros) {
 
 
 int64_t TimelineEventRecorder::TimeOriginMicros() const {
+  if (time_high_micros_ == 0) {
+    return 0;
+  }
   return time_low_micros_;
 }
 
 
 int64_t TimelineEventRecorder::TimeExtentMicros() const {
+  if (time_high_micros_ == 0) {
+    return 0;
+  }
   return time_high_micros_ - time_low_micros_;
 }