Only call OS::GetCurrentThreadCPUMicros when a TimelineDurationScope is enabled

runtime/vm/timeline.cc

Index: runtime/vm/timeline.cc
diff --git a/runtime/vm/timeline.cc b/runtime/vm/timeline.cc
index 54735a033f58610559c46398fb94394358fb0200..4eea19e91427cf9d831275bb2b97fe45104385b7 100644
--- a/runtime/vm/timeline.cc
+++ b/runtime/vm/timeline.cc
@@ -884,7 +884,7 @@ void TimelineEventScope::StealArguments(TimelineEvent* event) {
 TimelineDurationScope::TimelineDurationScope(TimelineStream* stream,
                                              const char* label)
     : TimelineEventScope(stream, label) {
-  if (!FLAG_support_timeline) {
+  if (!FLAG_support_timeline || !enabled()) {
     return;
   }
   timestamp_ = OS::GetCurrentMonotonicMicros();
@@ -896,7 +896,7 @@ TimelineDurationScope::TimelineDurationScope(Thread* thread,
                                              TimelineStream* stream,
                                              const char* label)
     : TimelineEventScope(thread, stream, label) {
-  if (!FLAG_support_timeline) {
+  if (!FLAG_support_timeline || !enabled()) {
     return;
   }
   timestamp_ = OS::GetCurrentMonotonicMicros();