Updating JSON Schema Compiler to add description field to schemas.

tools/json_schema_compiler/test/additional_properties.json

 [
   {
     "namespace": "additionalProperties",
+    "description": "The additionalProperties API.",
     "types": [
       {
         "id": "AdditionalPropertiesType",
         "type": "object",
         "properties": {
           "string": {
             "type": "string",
             "description": "Some string."
           }
         },
@@ skipping 32 matching lines @@
                 },
                 "additionalProperties": {"type": "string"}
               }
             ]
           }
         ]
       }
     ]
   }
 ]