[Extensions] Fix a check in the JSON Schema Compiler

There was an unnecessary check and graceful handling in the
JSON Schema Compiler that should never have been hit. Replace it with
an assert.

Bug: None

Change-Id: I5cfab6de46f042233ad66f98215e6737e3492afd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764103
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690097}

tools/json_schema_compiler/model.py

@@ -320,13 +320,11 @@ class Function(object):
     callback_param = None
     params = json.get('parameters', [])
     for i, param in enumerate(params):
-      if param.get('type') == 'function' and i == len(params) - 1:
-        if callback_param:
-          # No ParseException because the webstore has this.
-          # Instead, pretend all intermediate callbacks are properties.
-          self.params.append(GeneratePropertyFromParam(callback_param))
+      if i == len(params) - 1 and param.get('type') == 'function':
         callback_param = param
       else:
+        # Treat all intermediate function arguments as properties. Certain APIs,
+        # such as the webstore, have these.
         self.params.append(GeneratePropertyFromParam(param))
 
     if callback_param: