[tracing] Fix races in TraceLog's EnabledStateObserver

components/tracing/child_memory_dump_manager_delegate_impl.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_TRACING_CHILD_MEMORY_DUMP_MANAGER_DELEGATE_IMPL_H_
 #define COMPONENTS_TRACING_CHILD_MEMORY_DUMP_MANAGER_DELEGATE_IMPL_H_
 
 #include "base/trace_event/memory_dump_manager.h"
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/singleton.h"
+#include "base/synchronization/lock.h"
 
 namespace base {
 class SingleThreadTaskRunner;
 }  // namespace base
 
 namespace tracing {
 
 class ChildTraceMessageFilter;
 
 // This class is a simple proxy class between the MemoryDumpManager and the
@@ skipping 35 matching lines @@
 
   ChildMemoryDumpManagerDelegateImpl();
   ~ChildMemoryDumpManagerDelegateImpl() override;
 
   ChildTraceMessageFilter* ctmf_;  // Not owned.
 
   // The SingleThreadTaskRunner where the |ctmf_| lives.
   // 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_;
 
   DISALLOW_COPY_AND_ASSIGN(ChildMemoryDumpManagerDelegateImpl);
 };
 
 }  // namespace tracing
 
 #endif  // COMPONENTS_TRACING_CHILD_MEMORY_DUMP_MANAGER_DELEGATE_IMPL_H_