Added a tooltip for the change background gear head on the New Tab Page

R=ramyan@chromium.org

Bug: 882822
Change-Id: I1a70d25d39a927d0a07a2a77420f0b696513c788
Reviewed-on: https://chromium-review.googlesource.com/1241795Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Tanya Gupta <tgupta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593998}

chrome/app/generated_resources.grd

@@ -4793,7 +4793,7 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_NTP_CONFIRM_MSG_RESTORE_DEFAULTS" desc="The text label of the restore default shortcuts button when editing a custom link. (On the New Tab Page)">
         Restore default shortcuts
       </message>
-      <message name="IDS_NTP_CUSTOM_BG_CUSTOMIZE_BACKGROUND" desc="The title for the tooltip for background customization. (On the New Tab Page)">
+      <message name="IDS_NTP_CUSTOM_BG_CUSTOMIZE_BACKGROUND" desc="The title and aria-label for the tooltip for background customization. (On the New Tab Page)">
         Customize this page
       </message>
       <message name="IDS_NTP_CUSTOM_BG_GOOGLE_PHOTOS" desc="The text label of the Google Photos option for background customization. (On the New Tab Page)">

chrome/browser/resources/local_ntp/custom_backgrounds.js

@@ -902,6 +902,9 @@ customBackgrounds.init = function(showErrorNotification) {
       .setAttribute(
           'aria-label', configData.translatedStrings.customizeThisPage);
 
+  $(customBackgrounds.IDS.EDIT_BG_GEAR)
+      .setAttribute('title', configData.translatedStrings.customizeBackground);
+
   // Edit gear icon interaction events.
   let editBackgroundInteraction = function() {
     editDialog.showModal();