bindings: Use Visitor to collect union types

Source/bindings/scripts/idl_types.py

Index: Source/bindings/scripts/idl_types.py
diff --git a/Source/bindings/scripts/idl_types.py b/Source/bindings/scripts/idl_types.py
index 0db2514f58d815cc91bd884c38f4394855e71607..5d776a7172c563d978594851d86f0b59588d4288 100644
--- a/Source/bindings/scripts/idl_types.py
+++ b/Source/bindings/scripts/idl_types.py
@@ -124,6 +124,11 @@ class IdlTypeBase(object):
         raise NotImplementedError(
             'resolve_typedefs should be defined in subclasses')
 
+    def idl_types(self):
+        """A generator which yields IdlTypes which are referenced from |self|,
+        including itself."""
+        yield self
+
 
 ################################################################################
 # IdlType
@@ -321,6 +326,12 @@ class IdlUnionType(IdlTypeBase):
             for member_type in self.member_types]
         return self
 
+    def idl_types(self):
+        yield self
+        for member_type in self.member_types:
+            for idl_type in member_type.idl_types():
+                yield idl_type
+
 
 ################################################################################
 # IdlArrayOrSequenceType, IdlArrayType, IdlSequenceType
@@ -357,6 +368,11 @@ class IdlArrayOrSequenceType(IdlTypeBase):
     def enum_type(self):
         return self.element_type.enum_type
 
+    def idl_types(self):
+        yield self
+        for idl_type in self.element_type.idl_types():
+            yield idl_type
+
 
 class IdlArrayType(IdlArrayOrSequenceType):
     def __init__(self, element_type):
@@ -421,3 +437,8 @@ class IdlNullableType(IdlTypeBase):
     def resolve_typedefs(self, typedefs):
         self.inner_type = self.inner_type.resolve_typedefs(typedefs)
         return self
+
+    def idl_types(self):
+        yield self
+        for idl_type in self.inner_type.idl_types():
+            yield idl_type