Add touch highlighting to NTP shortcuts

Add the default selectable item background to the bookmarks/downloads
shortcuts on the "simplified NTP".

BUG=852392

Change-Id: Iebf2dce9d36d458d566a15ee2e9eeb8b1362bdee
Reviewed-on: https://chromium-review.googlesource.com/1105192Reviewed-by: Becky Zhou <huayinz@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568216}

chrome/android/java/res/layout/ntp_shortcuts.xml

@@ -21,6 +21,7 @@
         android:layout_weight="1"
         android:layout_height="wrap_content"
         android:minHeight="48dp"
+        android:background="?attr/selectableItemBackground"
         android:paddingTop="4dp"
         android:paddingBottom="4dp" >
 
@@ -44,6 +45,7 @@
         android:layout_weight="1"
         android:layout_height="wrap_content"
         android:minHeight="48dp"
+        android:background="?attr/selectableItemBackground"
         android:paddingTop="4dp"
         android:paddingBottom="4dp" >