Bug 1449806: Merge {Servo,Generic}SpecifiedValues into MappedDeclarations. r=xidorn

The idea would be that this header is only included in cpp files, thus it's ok
to include ServoBindings, etc.

MozReview-Commit-ID: EgQEsR0cZd4

dom/html/HTMLObjectElement.h

@@ -275,7 +275,7 @@ private:
   virtual JSObject* WrapNode(JSContext *aCx, JS::Handle<JSObject*> aGivenProto) override;
 
   static void MapAttributesIntoRule(const nsMappedAttributes* aAttributes,
-                                    GenericSpecifiedValues* aGenericData);
+                                    MappedDeclarations&);
 
   /**
    * This function is called by AfterSetAttr and OnAttrSetButNotChanged.