Bug 1246153 part 1. Restrict initialization of dictionaries from JSON to dictionaries that can actually be represented in JSON. r=bholley

dom/bindings/Codegen.py

@@ -12485,10 +12485,12 @@ class CGDictionary(CGThing):
             methods.append(self.initIdsMethod())
 
         methods.append(self.initMethod())
-        methods.append(self.initFromJSONMethod())
+        canBeRepresentedAsJSON = self.dictionarySafeToJSONify(self.dictionary)
+        if canBeRepresentedAsJSON:
+            methods.append(self.initFromJSONMethod())
         try:
             methods.append(self.toObjectInternalMethod())
-            if self.dictionarySafeToJSONify(self.dictionary):
+            if canBeRepresentedAsJSON:
                 methods.append(self.toJSONMethod())
         except MethodNotNewObjectError:
             # If we can't have a ToObjectInternal() because one of our members