Fix base::internal::WeakReferenceOwner races.

base/memory/weak_ptr.h

Index: base/memory/weak_ptr.h
diff --git a/base/memory/weak_ptr.h b/base/memory/weak_ptr.h
index 0c6b4a70b46f3b06ef2bfab6b94cb7bc1b3da5e8..225c195d599d1e156dbef9616f076e02b6847e15 100644
--- a/base/memory/weak_ptr.h
+++ b/base/memory/weak_ptr.h
@@ -55,6 +55,7 @@
 #include "base/base_api.h"
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
+#include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
 
 namespace base {
@@ -103,6 +104,7 @@ class BASE_API WeakReferenceOwner {
   WeakReference GetRef() const;
 
   bool HasRefs() const {
+    AutoLock auto_lock(lock_);
     return flag_ != NULL;
   }
 
@@ -110,10 +112,14 @@ class BASE_API WeakReferenceOwner {
 
   // Indicates that this object will be used on another thread from now on.
   void DetachFromThread() {
+    AutoLock auto_lock(lock_);
     if (flag_) flag_->DetachFromThread();
   }
 
  private:
+  // Needs locking since WeakReference::Flag may be destroyed on a different
+  // thread.
+  mutable Lock lock_;
   mutable WeakReference::Flag* flag_;
 };