Add the layout test case of ash/popup_message.

ash/popup_message.h

Index: ash/popup_message.h
diff --git a/ash/popup_message.h b/ash/popup_message.h
index 8731536401953654cb069d1674546b36582cd0cc..c2b160fa003e4553e388afa82e3004aae14410d2 100644
--- a/ash/popup_message.h
+++ b/ash/popup_message.h
@@ -7,6 +7,7 @@
 
 #include "ash/ash_export.h"
 #include "base/basictypes.h"
+#include "base/gtest_prod_util.h"
 #include "base/strings/string16.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/views/bubble/bubble_border.h"
@@ -62,8 +63,13 @@ class ASH_EXPORT PopupMessage {
   void Close();
 
  private:
+  FRIEND_TEST_ALL_PREFIXES(PopupMessageTest, Layout);
+
   class MessageBubble;
 
+  static const int kCaptionLabelID;
+  static const int kMessageLabelID;
+
   void CancelHidingAnimation();
 
   MessageBubble* view_;