Bug 857645. Make it possible to copy-initialize an nsCOMPtr<T> from an already_AddRefed<U> as long as U inherits from T. r=ehsan

xpcom/glue/nsCOMPtr.h

@@ -20,6 +20,8 @@
 */
 
 #include "mozilla/Attributes.h"
+#include "mozilla/TypeTraits.h"
+#include "mozilla/Assertions.h"
 
   // Wrapping includes can speed up compiles (see "Large Scale C++ Software Design")
 #ifndef nsDebug_h___
@@ -548,6 +550,18 @@ class nsCOMPtr MOZ_FINAL
           NSCAP_ASSERT_NO_QUERY_NEEDED();
         }
 
+      template<typename U>
+      nsCOMPtr( const already_AddRefed<U>& aSmartPtr )
+            : NSCAP_CTOR_BASE(static_cast<T*>(aSmartPtr.mRawPtr))
+          // construct from |dont_AddRef(expr)|
+        {
+          // But make sure that U actually inherits from T
+          MOZ_STATIC_ASSERT((mozilla::IsBaseOf<T, U>::value),
+                            "U is not a subclass of T");
+          NSCAP_LOG_ASSIGNMENT(this, static_cast<T*>(aSmartPtr.mRawPtr));
+          NSCAP_ASSERT_NO_QUERY_NEEDED();
+        }
+
       nsCOMPtr( const nsQueryInterface qi )
             : NSCAP_CTOR_BASE(0)
           // construct from |do_QueryInterface(expr)|