[NTP] Reset scroll position of background menu

When the menu is reset also reset the scroll position
to the top.

Bug: 937570
Change-Id: I9e90b986a26bfab5af88d481fa2c35afceba8c35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611771
Auto-Submit: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659683}

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -316,6 +316,7 @@ customBackgrounds.richerPicker_resetImageMenu = function(showMenu) {
   menu.classList.toggle(customBackgrounds.CLASSES.ON_IMAGE_MENU, false);
   backgroundMenu.classList.toggle(
       customBackgrounds.CLASSES.MENU_SHOWN, showMenu);
+  backgroundMenu.scrollTop = 0;
 
   // Reset done button state.
   $(customBackgrounds.IDS.MENU_DONE).disabled = true;