[NTP] Not close custom background dialog when clicking outside

Prevent closing the custom background dialog when clicking outside to
stay consistent with the custom link edit dialog. Also fix bug 874820.
When dragging mouse cursor within the overlay, now the dialog will not
close.

Bug: 874820
Change-Id: I4f78da252df73a664a7c31099e7e311a6c7856ed
Reviewed-on: https://chromium-review.googlesource.com/1185799Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585369}

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -1069,13 +1069,6 @@ customBackgrounds.initCustomBackgrounds = function() {
     }
   };
 
-  // Clicks that happen outside of the dialog.
-  menu.onclick = function(event) {
-    if (event.target == menu) {
-      customBackgrounds.closeCollectionDialog(menu);
-    }
-  };
-
   // Interactions with the back arrow on the image selection dialog.
   var backInteraction = function(event) {
     customBackgrounds.resetSelectionDialog();