base::RefCounted now DCHECKs when referenced from multiple threads.

base/memory/ref_counted.h

Index: base/memory/ref_counted.h
diff --git a/base/memory/ref_counted.h b/base/memory/ref_counted.h
index 219437ed8d82b8b35835a6c3297d5f719f14c277..4be15840371e3da4ee65193ee069d8909426e1fe 100644
--- a/base/memory/ref_counted.h
+++ b/base/memory/ref_counted.h
@@ -14,6 +14,7 @@
 #ifndef NDEBUG
 #include "base/logging.h"
 #endif
+#include "base/threading/thread_checker.h"
 #include "base/threading/thread_collision_warner.h"
 #include "build/build_config.h"
 
@@ -119,16 +120,21 @@ class BASE_EXPORT RefCountedThreadSafeBase {
 //
 // You should always make your destructor private, to avoid any code deleting
 // the object accidently while there are references to it.
+//
+// RefCounted objects are not thread-safe. To use a ref-counted object from more
+// than one thead, use a RefCountedThreadSafe.
 template <class T>
 class RefCounted : public subtle::RefCountedBase {
  public:
   RefCounted() {}
 
   void AddRef() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
     subtle::RefCountedBase::AddRef();
   }
 
   void Release() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
     if (subtle::RefCountedBase::Release()) {
       delete static_cast<const T*>(this);
     }
@@ -138,9 +144,34 @@ class RefCounted : public subtle::RefCountedBase {
   ~RefCounted() {}
 
  private:
+  base::ThreadChecker thread_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(RefCounted<T>);
 };
 
+// Same as RefCounted, but does not enforce thread safety. Subclasses of
+// UnsafeRefCounted probably have race conditions and should be fixed up, either
+// by ensuring they are only referenced from one thread (and moving to
+// RefCounted), or by moving to RefCountedThreadSafe.
+template <class T>
+class UnsafeRefCounted : public subtle::RefCountedBase {
+ public:
+  UnsafeRefCounted() {}
+
+  void AddRef() const { subtle::RefCountedBase::AddRef(); }
+
+  void Release() const {
+    if (subtle::RefCountedBase::Release())
+      delete static_cast<const T*>(this);
+  }
+
+ protected:
+  ~UnsafeRefCounted() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(UnsafeRefCounted<T>);
+};
+
 // Forward declaration.
 template <class T, typename Traits> class RefCountedThreadSafe;