Add build flag to disable hotwording.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index b26d0e1aaf3baa8c0bea2aa21795051e0767bb39..bac0544541b1ffbd4c9a072e86425476f01fa3d9 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -391,6 +391,9 @@
       # Web speech is enabled by default. Set to 0 to disable.
       'enable_web_speech%': 1,
 
+      # 'Ok Google' hotwording is enabled by default. Set to 0 to disable.
+      'enable_hotwording%': 1,
+
       # Notifications are compiled in by default. Set to 0 to disable.
       'notifications%' : 1,
 
@@ -1136,6 +1139,7 @@
     'configuration_policy%': '<(configuration_policy)',
     'safe_browsing%': '<(safe_browsing)',
     'enable_web_speech%': '<(enable_web_speech)',
+    'enable_hotwording%': '<(enable_hotwording)',
     'notifications%': '<(notifications)',
     'clang_use_chrome_plugins%': '<(clang_use_chrome_plugins)',
     'mac_want_real_dsym%': '<(mac_want_real_dsym)',