speech: Change SpeechInputBubbleBase::message_text() to return a const-ref.

chrome/browser/speech/speech_input_bubble.h

Index: chrome/browser/speech/speech_input_bubble.h
diff --git a/chrome/browser/speech/speech_input_bubble.h b/chrome/browser/speech/speech_input_bubble.h
index d2c598e3d8257c4d901911a42a24c4f5c4d2e679..9267f2015e1610037d13c1710676fb2ac29024a9 100644
--- a/chrome/browser/speech/speech_input_bubble.h
+++ b/chrome/browser/speech/speech_input_bubble.h
@@ -152,13 +152,9 @@ class SpeechInputBubbleBase : public SpeechInputBubble {
   // Overridden by subclasses to copy |icon_image()| to the screen.
   virtual void UpdateImage() = 0;
 
-  DisplayMode display_mode() {
-    return display_mode_;
-  }
+  DisplayMode display_mode() const { return display_mode_; }
 
-  string16 message_text() {
-    return message_text_;
-  }
+  const string16& message_text() const { return message_text_; }
 
   SkBitmap icon_image();