[NTP] Fix RTL arrow key navigation

Previously arrow key navigation was fixed for the collection
selection dialog in RTL mode, but not for the image
selection dialog.

Bug: 880764
Change-Id: I96f4f007d6f8f8ecda7a5f72e8353e75a6391211
Reviewed-on: https://chromium-review.googlesource.com/1220767Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590573}

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -716,11 +716,15 @@ customBackgrounds.showImageSelectionDialog = function(dialogTitle) {
 
         let target = null;
         if (event.keyCode == customBackgrounds.KEYCODES.LEFT) {
-          target = customBackgrounds.getNextTile(-1, 0, this.dataset.tile_num);
+          target = customBackgrounds.getNextTile(
+              document.documentElement.classList.contains('rtl') ? 1 : -1, 0,
+              this.dataset.tile_num);
         } else if (event.keyCode == customBackgrounds.KEYCODES.UP) {
           target = customBackgrounds.getNextTile(0, -1, this.dataset.tile_num);
         } else if (event.keyCode == customBackgrounds.KEYCODES.RIGHT) {
-          target = customBackgrounds.getNextTile(1, 0, this.dataset.tile_num);
+          target = customBackgrounds.getNextTile(
+              document.documentElement.classList.contains('rtl') ? -1 : 1, 0,
+              this.dataset.tile_num);
         } else if (event.keyCode == customBackgrounds.KEYCODES.DOWN) {
           target = customBackgrounds.getNextTile(0, 1, this.dataset.tile_num);
         }