Add ability to generate typedefs from dawn.json

Can be used to help with deprecation during simple struct renames.

Includes typedefs for VertexAttributeDescriptor -> VertexAttribute and
VertexBufferLayoutDescriptor -> VertexBufferLayout as specified by the
latest RenderPipelineDescriptor changes.

Bug: dawn:642
Change-Id: Iab3d74d179884499540e813b0e66859713031ccb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/40581
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

generator/dawn_json_generator.py

@@ -128,6 +128,12 @@ class CallbackType(Type):
         self.arguments = []
 
 
+class TypedefType(Type):
+    def __init__(self, name, json_data):
+        Type.__init__(self, name, json_data)
+        self.type = None
+
+
 class NativeType(Type):
     def __init__(self, name, json_data):
         Type.__init__(self, name, json_data, native=True)
@@ -271,6 +277,10 @@ def link_callback(callback, types):
                                                types)
 
 
+def link_typedef(typedef, types):
+    typedef.type = types[typedef.json_data['type']]
+
+
 # Sort structures so that if struct A has struct B as a member, then B is
 # listed before A.
 #
@@ -321,6 +331,7 @@ def parse_json(json):
         'callback': CallbackType,
         'object': ObjectType,
         'structure': StructureType,
+        'typedef': TypedefType,
     }
 
     types = {}
@@ -346,6 +357,9 @@ def parse_json(json):
     for callback in by_category['callback']:
         link_callback(callback, types)
 
+    for typedef in by_category['typedef']:
+        link_typedef(typedef, types)
+
     for category in by_category.keys():
         by_category[category] = sorted(
             by_category[category], key=lambda typ: typ.name.canonical_case())