Cleanup: Pass std::string as const reference if possible

chrome/common/icon_with_badge_image_source.h

Index: chrome/common/icon_with_badge_image_source.h
diff --git a/chrome/common/icon_with_badge_image_source.h b/chrome/common/icon_with_badge_image_source.h
index 3270ce3244dd5095f35c2f74c3f51a8d347d4c82..77331fbf673e7bcfaaf6348d509934a5a84461eb 100644
--- a/chrome/common/icon_with_badge_image_source.h
+++ b/chrome/common/icon_with_badge_image_source.h
@@ -22,7 +22,9 @@ class IconWithBadgeImageSource : public gfx::CanvasImageSource {
  public:
   // The data representing a badge to be painted over the base image.
   struct Badge {
-    Badge(std::string text, SkColor text_color, SkColor background_color);
+    Badge(const std::string& text,
+          SkColor text_color,
+          SkColor background_color);
     ~Badge();
 
     std::string text;