Fix up a few more lint warnings in chrome/

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

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2014 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.
 -->
 
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content">
@@ skipping 43 matching lines @@
                 android:layout_marginStart="16dp"
                 android:ems="4"
                 android:hint="@string/card_unmask_input_hint" />
 
             <ImageView
                 android:id="@+id/cvc_hint_image"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:layout_gravity="center_vertical"
                 android:layout_marginStart="8dp"
-                android:layout_marginEnd="16dp" />
+                android:layout_marginEnd="16dp"
+                android:contentDescription="@null" />
         </LinearLayout>
     </LinearLayout>
 
     <LinearLayout
         android:id="@+id/verification_overlay"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignTop="@+id/main_contents"
         android:layout_alignBottom="@+id/main_contents"
         android:layout_alignStart="@+id/main_contents"
         android:layout_alignEnd="@+id/main_contents"
         android:orientation="vertical"
         android:gravity="center"
         android:visibility="gone">
 
         <!-- TODO(estade): should be material styled. -->
         <ProgressBar
             style="@android:style/Widget.ProgressBar.Small"
             android:id="@+id/verification_progress_bar"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_marginBottom="8dp"
             android:layout_marginTop="30dp"
             android:visibility="gone" />
 
+        <!--  TODO(estade): add a real content description for this image. -->
         <ImageView
             android:id="@+id/verification_success"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:src="@drawable/verify_checkmark"
-            android:visibility="gone" />
+            android:visibility="gone"
+            android:contentDescription="@null" />
 
         <TextView
             android:id="@+id/verification_message"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:textColor="?android:attr/colorAccent"
             android:textStyle="bold" />
     </LinearLayout>
 </RelativeLayout>