[NTP] Remove attribution section after restore default background

Attribution section should be removed after clicking restore default
background. Adding clearAttribution function when the
restoreDefaultInteraction is called.

Screenshot:
https://screenshot.googleplex.com/AQT8eBddJ7A.png

Bug: 869693
Change-Id: I18526c72c74fbff84320faf5f8a756c545138fcf
Reviewed-on: https://chromium-review.googlesource.com/1157650Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579871}

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -843,6 +843,7 @@ customBackgrounds.initCustomBackgrounds = function() {
   // Interactions with the "Restore default background" option.
   var restoreDefaultInteraction = function(event) {
     editDialog.close();
+    customBackgrounds.clearAttribution();
     window.chrome.embeddedSearch.newTabPage.setBackgroundURL('');
   };
   $(customBackgrounds.IDS.RESTORE_DEFAULT).onclick = restoreDefaultInteraction;