Add build flag to disable hotwording.

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index f3e16ecde8ad6a9a703e8a17861eeee4691e4004..57f3493f7ccd05a86511b236e1a927732ac39c13 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -19,6 +19,11 @@ 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"
@@ -451,6 +456,10 @@ source_set("browser") {
     }
   }
 
+  if (enable_hotwording) {
+    defines += [ "ENABLE_HOTWORDING" ]
+  }
+
   if (is_linux) {
     deps += [
       "//device/media_transfer_protocol",