[NTP] Allow space to be used on the back arrow

In https://crrev.com/c/1249834 the tabbable element was changed
from a button to role=button so using space to press it no longer
worked.

Bug: 891652
Change-Id: I8dd7155de5deba6f2b25c3ba5cb39761f84eafdc
Reviewed-on: https://chromium-review.googlesource.com/c/1258880Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596396}

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -1222,9 +1222,10 @@ customBackgrounds.initCustomBackgrounds = function(showErrorNotification) {
     customBackgrounds.showCollectionSelectionDialog(
         customBackgrounds.dialogCollectionsSource);
   };
-  $(customBackgrounds.IDS.BACK).onclick = backInteraction;
+  $(customBackgrounds.IDS.BACK_CIRCLE).onclick = backInteraction;
   $(customBackgrounds.IDS.BACK_CIRCLE).onkeyup = function(event) {
-    if (event.keyCode === customBackgrounds.KEYCODES.ENTER) {
+    if (event.keyCode === customBackgrounds.KEYCODES.ENTER ||
+        event.keyCode === customBackgrounds.KEYCODES.SPACE) {
       backInteraction(event);
     }
   };