Disable "Ok Google" hotwording in open source builds by default.

chrome/browser/BUILD.gn

 # Copyright 2014 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("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//third_party/protobuf/proto_library.gni")
 
 # //build/config/android/rules.gni imports //tools/grit/grit_rule.gni, which
 # produces a conflict for the "grit" template so we have to only include one.
 if (is_android) {
   import("//build/config/android/rules.gni")
 } else {
   import("//tools/grit/grit_rule.gni")
 }
 if (is_desktop_linux) {
   import("//build/config/linux/pkg_config.gni")
 }
 
 declare_args() {
-  # 'Ok Google' hotwording is enabled.
-  enable_hotwording = true
+  # 'Ok Google' hotwording is disabled by default in open source builds. Set to
+  # true to enable. (This will download a closed-source NaCl module at startup.)
+  # Chrome-branded builds have this enabled by default.
+  enable_hotwording = is_chrome_branded
 }
 
 about_credits_file = "$target_gen_dir/about_credits.html"
 additional_modules_list_file =
     "$root_gen_dir/chrome/browser/internal/additional_modules_list.txt"
 
 gypi_values = exec_script("//build/gypi_to_gn.py",
                           [ rebase_path("../chrome_browser.gypi") ],
                           "scope",
                           [ "../chrome_browser.gypi" ])
@@ skipping 1139 matching lines @@
     ]
   }
 
   if (enable_wifi_bootstrapping) {
     sources += [
       "local_discovery/wifi/mock_wifi_manager.cc",
       "local_discovery/wifi/mock_wifi_manager.h",
     ]
   }
 }