[tracing] Fix races in TraceLog's EnabledStateObserver

components/tracing/child_memory_dump_manager_delegate_impl.h

Index: components/tracing/child_memory_dump_manager_delegate_impl.h
diff --git a/components/tracing/child_memory_dump_manager_delegate_impl.h b/components/tracing/child_memory_dump_manager_delegate_impl.h
index 099948e64185bd5edf20d0865fd33dab1bde1f28..ca5de293ed7f233cd06dcd286806c8161bd97b08 100644
--- a/components/tracing/child_memory_dump_manager_delegate_impl.h
+++ b/components/tracing/child_memory_dump_manager_delegate_impl.h
@@ -9,6 +9,7 @@
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/singleton.h"
+#include "base/synchronization/lock.h"
 
 namespace base {
 class SingleThreadTaskRunner;
@@ -64,6 +65,10 @@ class ChildMemoryDumpManagerDelegateImpl
   // It is NULL iff |cmtf_| is NULL.
   scoped_refptr<base::SingleThreadTaskRunner> ctmf_task_runner_;
 
+  // Protects from concurrent access to |ctmf_task_runner_| to allow
+  // RequestGlobalMemoryDump to be called from arbitrary threads.
+  base::Lock lock_;
+
   // The unique id of the child process, created for tracing and is expected to
   // be valid only when tracing is enabled.
   uint64 tracing_process_id_;