Do not pass use_prebuilt_mojo_shell to gn.

scripts/slave/recipe_modules/chromium/config.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import pipes
 
 from slave.recipe_config import config_item_context, ConfigGroup
 from slave.recipe_config import Dict, List, Single, Static, Set, BadConf
 from slave.recipe_config_types import Path
 
@@ skipping 468 matching lines @@
   dcheck(c, optional=True)
 
 @config_ctx()
 def gn_component_build(c):
   c.gn_args.append('is_component_build=true')
 
 @config_ctx()
 def gn_minimal_symbols(c):
   c.gn_args.append('symbol_level=1')
 
-@config_ctx()
-def gn_use_prebuilt_mojo_shell(c):
-  c.gn_args.append('use_prebuilt_mojo_shell=true')
-
 #### 'Full' configurations
 @config_ctx(includes=['ninja', 'default_compiler'])
 def chromium_no_goma(c):
   c.compile_py.default_targets = ['All', 'chromium_builder_tests']
 
 @config_ctx(includes=['ninja', 'default_compiler', 'goma'])
 def chromium(c):
   c.compile_py.default_targets = ['All', 'chromium_builder_tests']
 
 @config_ctx(includes=['ninja', 'clang'])  # Intentionally no goma yet.
@@ skipping 242 matching lines @@
 def chromium_perf_fyi(c):
   c.compile_py.clobber = False
   if c.HOST_PLATFORM == 'win':
     c.compile_py.compiler = None
     c.compile_py.goma_dir = None
     c.gyp_env.GYP_DEFINES['use_goma'] = 0
 
 @config_ctx()
 def chromium_deterministic_build(c):
   c.gyp_env.GYP_DEFINES['dont_embed_build_metadata'] = 1