Make timeline use the same clock source as mojo's tracing infrastructure

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index f6a255d621ad024d36b7678f59353ef493e9bf7b..f25add71d282187f1f66be75c881ed6fe3293c13 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -308,6 +308,11 @@ void JSONStream::PrintValueTimeMillis(int64_t millis) {
 }
 
 
+void JSONStream::PrintValueTimeMicros(int64_t micros) {
+  PrintValue64(micros);
+}
+
+
 void JSONStream::PrintValue(double d) {
   PrintCommaIfNeeded();
   buffer_.Printf("%f", d);
@@ -465,6 +470,11 @@ void JSONStream::PrintPropertyTimeMillis(const char* name, int64_t millis) {
 }
 
 
+void JSONStream::PrintPropertyTimeMicros(const char* name, int64_t micros) {
+  PrintProperty64(name, micros);
+}
+
+
 void JSONStream::PrintProperty(const char* name, double d) {
   PrintPropertyName(name);
   PrintValue(d);