Add language_prefs namespace to language_preferences.h.

chrome/browser/chromeos/status/language_menu_button.cc

Index: chrome/browser/chromeos/status/language_menu_button.cc
diff --git a/chrome/browser/chromeos/status/language_menu_button.cc b/chrome/browser/chromeos/status/language_menu_button.cc
index 5b042a7988d5f352895fecf36a8ba625e5d8676e..6e89c3d162b9edefc0868e35083a3e54a961da9f 100644
--- a/chrome/browser/chromeos/status/language_menu_button.cc
+++ b/chrome/browser/chromeos/status/language_menu_button.cc
@@ -441,7 +441,7 @@ void LanguageMenuButton::InputMethodChanged(InputMethodLibrary* obj) {
     // buttun for the login screen is destroyed.
     if (!logged_in_ && g_browser_process && g_browser_process->local_state()) {
         g_browser_process->local_state()->SetString(
-            kPreferredKeyboardLayout, current_input_method.id);
+            language_prefs::kPreferredKeyboardLayout, current_input_method.id);
         g_browser_process->local_state()->SavePersistentPrefs();
     }
   }
@@ -654,7 +654,8 @@ std::wstring LanguageMenuButton::GetTextForMenu(
 }
 
 void LanguageMenuButton::RegisterPrefs(PrefService* local_state) {
-  local_state->RegisterStringPref(kPreferredKeyboardLayout, "");
+  local_state->RegisterStringPref(language_prefs::kPreferredKeyboardLayout,
+                                  "");
 }
 
 void LanguageMenuButton::GetAmbiguousLanguageCodeSet(