Update from https://crrev.com/327068

base/synchronization/cancellation_flag.h

 // Copyright (c) 2011 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 BASE_SYNCHRONIZATION_CANCELLATION_FLAG_H_
 #define BASE_SYNCHRONIZATION_CANCELLATION_FLAG_H_
 
 #include "base/base_export.h"
 #include "base/atomicops.h"
 #include "base/threading/platform_thread.h"
@@ skipping 11 matching lines @@
 #if !defined(NDEBUG)
     set_on_ = PlatformThread::CurrentId();
 #endif
   }
   ~CancellationFlag() {}
 
   // Set the flag. May only be called on the thread which owns the object.
   void Set();
   bool IsSet() const;  // Returns true iff the flag was set.
 
+  // For subtle reasons that may be different on different architectures,
+  // a different thread testing IsSet() may erroneously read 'true' after
+  // this method has been called.
+  void UnsafeResetForTesting();
+
  private:
   base::subtle::Atomic32 flag_;
 #if !defined(NDEBUG)
   PlatformThreadId set_on_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(CancellationFlag);
 };
 
 }  // namespace base
 
 #endif  // BASE_SYNCHRONIZATION_CANCELLATION_FLAG_H_