Bug 1426494: followup: Add a missing forward declaration on a CLOSED TREE. r=me

MozReview-Commit-ID: JMBpLSEcUt6

dom/base/FuzzingFunctions.h

@@ -8,6 +8,9 @@
 #define mozilla_dom_FuzzingFunctions
 
 namespace mozilla {
+
+class ErrorResult;
+
 namespace dom {
 
 class GlobalObject;