Fix index changed when no image in the directory in gallery

ui/file_manager/gallery/js/slide_mode.js

Index: ui/file_manager/gallery/js/slide_mode.js
diff --git a/ui/file_manager/gallery/js/slide_mode.js b/ui/file_manager/gallery/js/slide_mode.js
index 54ccfc190836c795c1f1bde488583cff4d7f9a5f..ca53ea15b702978d678a5f0d161586842f103c48 100644
--- a/ui/file_manager/gallery/js/slide_mode.js
+++ b/ui/file_manager/gallery/js/slide_mode.js
@@ -918,6 +918,8 @@ SlideMode.prototype.getNextSelectedIndex_ = function(direction) {
         this.savedSelection_.length);
     return this.savedSelection_[pos];
   } else {
+    if(this.getSelectedIndex() == -1)
+      return -1;

[harukam1, 2016/08/09 07:18:57]

"slideshowButton" icon shows because selectedIndex can be change by hitting
right/left button, and "selectionModel_.selectedIndexes.length" become 1. (see
ui/file_manager/gallery/js/gallery.js, line 820-822)

I'm not exactly sure where to add this two lines.
Do you think it's unnatural to return -1 in get"Next"SelectedIndex_?

[fukino, 2016/08/09 07:49:18]

It's weird that selectionModel_.selectedIndexes.length returns 1 when there is
no item.
Could you check the code in Gallery where we remove the images?
I guess this issue can be fixed by clearing the selection model when we remove
all items.

[harukam1, 2016/08/10 04:34:47]

We have decided to modify advanceWithKeyboard method per offline discussion.

     return advance(this.getSelectedIndex(), this.getItemCount_());
   }
 };