Add contentDescription to offline badges on the NTP

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

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2015 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file. -->
 
 <!-- A most visited item, for display on the new tab page. -->
 <org.chromium.chrome.browser.ntp.MostVisitedItemView
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:chrome="http://schemas.android.com/apk/res-auto"
     android:layout_width="@dimen/most_visited_tile_width"
@@ skipping 10 matching lines @@
         android:layout_marginTop="12dp"
         android:contentDescription="@null" />
 
     <org.chromium.chrome.browser.widget.TintedImageView
         android:id="@+id/offline_badge"
         android:layout_width="@dimen/most_visited_offline_badge_size"
         android:layout_height="@dimen/most_visited_offline_badge_size"
         android:layout_marginStart="48dp"
         android:visibility="gone"
         android:background="@drawable/offline_badge_background"
-        android:contentDescription="@null"
+        android:contentDescription="@string/accessibility_ntp_offline_badge"
         chrome:tint="@color/most_visited_offline_badge_tint"
         android:src="@drawable/offline_bolt" />
 
     <View
         android:layout_width="@dimen/most_visited_icon_size"
         android:layout_height="@dimen/most_visited_icon_size"
         android:layout_marginStart="12dp"
         android:layout_marginEnd="12dp"
         android:layout_marginTop="12dp"
         android:background="@drawable/most_visited_item_highlight" />
 
     <TextView
         android:id="@+id/most_visited_title"
         android:layout_width="72dp"
         android:layout_height="wrap_content"
         android:layout_marginTop="66dp"
         android:ellipsize="end"
         android:gravity="center_horizontal"
         android:lines="2"
         android:lineSpacingMultiplier="1.17"
         android:textColor="@color/most_visited_text"
         android:textSize="12sp" />
 
 </org.chromium.chrome.browser.ntp.MostVisitedItemView>