Add Now setting and migration logic.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 9aea51b666f6bc6ef7a6b7d429dffe18a83abf6d..451d1cbfacd30282be6c18f7e08c6e1dd3a49561 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -1673,6 +1673,10 @@ const char kGoogleGeolocationAccessEnabled[] =
     "googlegeolocationaccess.enabled";
 #endif
 
+// Boolean that specifies whether to enable the Google Now Launcher extension.
+// Note: This is not the notifications component gated by ENABLE_GOOGLE_NOW.
+const char kGoogleNowLauncherEnabled[] = "google_now_launcher.enabled";
+
 // The default audio capture device used by the Media content setting.
 const char kDefaultAudioCaptureDevice[] = "media.default_audio_capture_device";