Add build flag to disable hotwording.

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 disabled by default in Chromium builds.
+  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" ])
 
 if (is_desktop_linux) {
@@ skipping 412 matching lines @@
                       "//chrome")
       deps += [ "//chrome/common/safe_browsing:proto" ]
     } else if (safe_browsing_mode == 3) {
       sources += rebase_path(
               gypi_values.chrome_browser_safe_browsing_mobile_extended_sources,
               ".",
               "//chrome")
     }
   }
 
+  if (enable_hotwording) {
+    defines += [ "ENABLE_HOTWORDING" ]
+  }
+
   if (is_linux) {
     deps += [
       "//device/media_transfer_protocol",
       "//device/udev_linux",
     ]
   }
   if (is_linux && !is_chromeos) {
     deps += [ "//third_party/speech-dispatcher" ]
   }
 
@@ skipping 699 matching lines @@
     ]
   }
 
   if (enable_wifi_bootstrapping) {
     sources += [
       "local_discovery/wifi/mock_wifi_manager.cc",
       "local_discovery/wifi/mock_wifi_manager.h",
     ]
   }
 }