Bug 1250985 - Part 1: Add a way to identify whether a WebIDL dictionary has any members present; r=bzbarsky

dom/bindings/BindingDeclarations.h

@@ -50,11 +50,19 @@ protected:
   struct FastDictionaryInitializer {
   };
 
+  bool mIsAnyMemberPresent = false;
+
 private:
   // aString is expected to actually be an nsAString*.  Should only be
   // called from StringifyToJSON.
   static bool AppendJSONToString(const char16_t* aJSONData,
                                  uint32_t aDataLength, void* aString);
+
+public:
+  bool IsAnyMemberPresent() const
+  {
+    return mIsAnyMemberPresent;
+  }
 };
 
 // Struct that serves as a base class for all typed arrays and array buffers and

dom/bindings/Codegen.py

@@ -12530,6 +12530,10 @@ class CGDictionary(CGThing):
             "convert": string.Template(conversionInfo.template).substitute(replacements),
             "propGet": propGet
         }
+        # The conversion code will only run where a default value or a value passed
+        # by the author needs to get converted, so we can remember if we have any
+        # members present here.
+        conversionReplacements["convert"] += "mIsAnyMemberPresent = true;\n"
         conversion = ("if (!isNull && !${propGet}) {\n"
                       "  return false;\n"
                       "}\n")