Make launching apps from shelf more intuitive

ash/shelf/wm_shelf.h

Index: ash/shelf/wm_shelf.h
diff --git a/ash/shelf/wm_shelf.h b/ash/shelf/wm_shelf.h
index 491f35e1b3ae4369371787afdf5671ffad546928..2c4b01a48bf171c9d45b387ea942d7d936fb65e1 100644
--- a/ash/shelf/wm_shelf.h
+++ b/ash/shelf/wm_shelf.h
@@ -125,6 +125,10 @@ class ASH_EXPORT WmShelf : public ShelfLayoutManagerObserver {
   // Activates the shelf item specified by the index in the list of shelf items.
   static void ActivateShelfItem(int item_index);

[stevenjb, 2017/05/22 17:55:28]

Is this still called? Maybe note where/why we do not always call 
ActivateShelfItemInDisplay? (Or if there are not a lot of examples, maybe just
remove this and pass display::kInvalidDisplayId in those instances).

[weidongg, 2017/05/23 16:37:46]

There are 40 occurrences of this function, so should I do the refactor like
removing ActivateShelfItemInDisplay and adding display_id parameter to
ActivateShelfItem?

 
+  // Activates the shelf item specified by the index in the list of shelf items
+  // in the display identified by |display_id|.

[Mr4D (OOO till 08-26), 2017/05/22 15:02:28]

on the display ...?

[weidongg, 2017/05/23 16:37:46]

Done.

+  static void ActivateShelfItemInDisplay(int item_index, int64_t display_id);
+
   // Handles a gesture |event| coming from a source outside the shelf widget
   // (e.g. the status area widget). Allows support for behaviors like toggling
   // auto-hide with a swipe, even if that gesture event hits another window.