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

runtime/vm/os_thread_linux.cc

Index: runtime/vm/os_thread_linux.cc
diff --git a/runtime/vm/os_thread_linux.cc b/runtime/vm/os_thread_linux.cc
index f9bb9b551cbf824239e6b6af9e6ab79ec88d84d0..4955cb46aa73576b6715d02aeedc7d310f9c3857 100644
--- a/runtime/vm/os_thread_linux.cc
+++ b/runtime/vm/os_thread_linux.cc
@@ -9,6 +9,7 @@
 
 #include <errno.h>  // NOLINT
 #include <sys/resource.h>  // NOLINT
+#include <sys/syscall.h>  // NOLINT
 #include <sys/time.h>  // NOLINT
 
 #include "platform/assert.h"
@@ -157,6 +158,11 @@ ThreadId OSThread::GetCurrentThreadId() {
 }
 
 
+ThreadId OSThread::GetCurrentThreadTraceId() {
+  return syscall(__NR_gettid);
+}
+
+
 ThreadJoinId OSThread::GetCurrentThreadJoinId() {
   return pthread_self();
 }