Fix index changed when no image in the directory in gallery

ui/file_manager/gallery/js/slide_mode.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Slide mode displays a single image and has a set of controls to navigate
  * between the images and to edit an image.
  *
  * @param {!HTMLElement} container Main container element.
  * @param {!HTMLElement} content Content container element.
@@ skipping 900 matching lines @@
   }
 
   // If the saved selection is multiple the Slideshow should cycle through
   // the saved selection.
   if (this.isSlideshowOn_() &&
       this.savedSelection_ && this.savedSelection_.length > 1) {
     var pos = advance(this.savedSelection_.indexOf(this.getSelectedIndex()),
         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_());
   }
 };
 
 /**
  * Advance the selection based on the pressed key ID.
  * @param {string} keyID Key of the KeyboardEvent.
  */
 SlideMode.prototype.advanceWithKeyboard = function(keyID) {
   var prev = (keyID === 'ArrowUp' ||
@@ skipping 1202 matching lines @@
 /**
  * Handles mouse up events.
  * @param {!Event} event Wheel event.
  * @private
  */
 TouchHandler.prototype.onMouseUp_ = function(event) {
   if (event.button !== 0)
     return;
   this.clickStarted_ = false;
 };