Bug 1463299: Add RefPtr::forget special case for ambiguous nsISupport…

…s casts. r=bz

When we use RefPtr with nsISupports sub-classes, it's usually because the type
cannot be unambiguously cast to nsISupports. We already have a ToSupports
generic function to resolve ambiguity in these cases, so we may as well use
it here.

MozReview-Commit-ID: FaHhPKAPn6j

--HG--
extra : rebase_source : dd8f3707bdebedfe559aed0caf2c3b0c49163735
extra : histedit_source : 9805787a169329b9c739dfa456cfe6a61a22b7d7

mfbt/RefPtr.h

@@ -17,6 +17,7 @@
 
 class nsQueryReferent;
 class nsCOMPtr_helper;
+class nsISupports;
 
 namespace mozilla {
 template<class T> class OwningNonNull;
@@ -277,6 +278,14 @@ class MOZ_IS_REFPTR RefPtr
     mRawPtr = nullptr;
   }
 
+  void
+  forget(nsISupports** aRhs)
+  {
+    MOZ_ASSERT(aRhs, "Null pointer passed to forget!");
+    *aRhs = ToSupports(mRawPtr);
+    mRawPtr = nullptr;
+  }
+
   T*
   get() const
   /*

widget/nsBaseFilePicker.cpp

@@ -53,7 +53,8 @@ LocalFileToDirectoryOrBlob(nsPIDOMWindowInner* aWindow,
     return NS_OK;
   }
 
-  *aResult = ToSupports(File::CreateFromFile(aWindow, aFile).take());
+  RefPtr<File> file = File::CreateFromFile(aWindow, aFile);
+  file.forget(aResult);
   return NS_OK;
 }