Add OSThread::GetCurrentThreadTraceId and use it in the timeline implementation

runtime/vm/os_thread_macos.cc

Index: runtime/vm/os_thread_macos.cc
diff --git a/runtime/vm/os_thread_macos.cc b/runtime/vm/os_thread_macos.cc
index ffb94e99fe559cf5a1ebecdc9e31ac7803f117f0..dbcd9e04fcd0b7e02880fd734bd9ff4b1176f081 100644
--- a/runtime/vm/os_thread_macos.cc
+++ b/runtime/vm/os_thread_macos.cc
@@ -150,6 +150,11 @@ ThreadId OSThread::GetCurrentThreadId() {
 }
 
 
+ThreadId OSThread::GetCurrentThreadTraceId() {
+  return pthread_mach_thread_np(pthread_self());
+}
+
+
 ThreadJoinId OSThread::GetCurrentThreadJoinId() {
   return pthread_self();
 }