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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 8eadaedec9efb67859d1fc402bd66c82b351e6d8..b55cdb0719863064229830e43c8b88279b42fd47 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -26,6 +26,7 @@
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/os_thread.h"
+#include "vm/os.h"
 #include "vm/port.h"
 #include "vm/precompiler.h"
 #include "vm/profiler.h"
@@ -5667,6 +5668,11 @@ DART_EXPORT Dart_Handle Dart_ServiceSendDataEvent(const char* stream_id,
 }
 
 
+DART_EXPORT int64_t Dart_TimelineGetMicros() {
+  return OS::GetCurrentTraceMicros();
+}
+
+
 DART_EXPORT void Dart_TimelineSetRecordedStreams(int64_t stream_mask) {
   Isolate* isolate = Isolate::Current();
   CHECK_ISOLATE(isolate);