[DevTools] Turn starting tracing into promise.

third_party/WebKit/Source/devtools/scripts/build/generate_protocol_externs.py

 #!/usr/bin/env python
 # Copyright (c) 2011 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 35 matching lines @@
 
 # yapf: disable
 promisified_domains = {
     "Accessibility",
     "Animation",
     "Browser",
     "CSS",
     "Emulation",
     "HeapProfiler",
     "Profiler",
-    "LayerTree"
+    "LayerTree",
+    "Tracing"
 }
 # yapf: enable
 
 ref_types = {}
 
 
 def full_qualified_type_id(domain_name, type_id):
     if type_id.find(".") == -1:
         return "%s.%s" % (domain_name, type_id)
     return type_id
@@ skipping 194 matching lines @@
     import sys
     import os.path
     program_name = os.path.basename(__file__)
     if len(sys.argv) < 5 or sys.argv[1] != "-o":
         sys.stderr.write("Usage: %s -o OUTPUT_FILE INPUT_FILE_1 INPUT_FILE_2\n" % program_name)
         exit(1)
     output_path = sys.argv[2]
     input_path_1 = sys.argv[3]
     input_path_2 = sys.argv[4]
     generate_protocol_externs(output_path, input_path_1, input_path_2)