Add ability to generate constants from dawn.json

The "constant" category has two keys:
 - `"type"`: a string, the name of the base data type
 - `"value"`: a string, the value is defined with preprocessor macro

Remove deprecated constant InputStepMode.

BUG=dawn:1201, dawn:1023

Change-Id: If4308bd5b25dddd9453514ce362bebe4fd771a57
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/70704
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>

generator/dawn_json_generator.py

@@ -239,6 +239,14 @@ class StructureType(Record, Type):
         return self.chained == "out" or self.extensible == "out"
 
 
+class ConstantDefinition():
+    def __init__(self, is_enabled, name, json_data):
+        self.type = None
+        self.value = json_data['value']
+        self.json_data = json_data
+        self.name = Name(name)
+
+
 class Command(Record):
     def __init__(self, name, members=None):
         Record.__init__(self, name)
@@ -310,6 +318,11 @@ def link_typedef(typedef, types):
     typedef.type = types[typedef.json_data['type']]
 
 
+def link_constant(constant, types):
+    constant.type = types[constant.json_data['type']]
+    assert constant.type.name.native
+
+
 # Sort structures so that if struct A has struct B as a member, then B is
 # listed before A.
 #
@@ -362,6 +375,7 @@ def parse_json(json, enabled_tags):
         'object': ObjectType,
         'structure': StructureType,
         'typedef': TypedefType,
+        'constant': ConstantDefinition,
     }
 
     types = {}
@@ -390,6 +404,9 @@ def parse_json(json, enabled_tags):
     for typedef in by_category['typedef']:
         link_typedef(typedef, types)
 
+    for constant in by_category['constant']:
+        link_constant(constant, types)
+
     for category in by_category.keys():
         by_category[category] = sorted(
             by_category[category], key=lambda typ: typ.name.canonical_case())