Address NewApi and other Android lint warnings in src/chrome.

chrome/android/java/res/layout/homepage_preferences.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. -->
 
 <ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:fillViewport="true" >
 
@@ skipping 24 matching lines @@
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:textOn="@string/text_on"
                 android:textOff="@string/text_off" />
 
         </LinearLayout>
 
         <ImageView
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
+            android:contentDescription="@null"
             android:src="?android:attr/listDivider"
             android:scaleType="fitXY" />
 
         <EditText
             android:id="@+id/custom_uri"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:layout_marginTop="16dp"
             android:layout_marginStart="16dp"
             android:layout_marginEnd="16dp"
             android:hint="@string/options_startup_pages_placeholder"
             android:inputType="textUri"
             android:nextFocusLeft="@id/custom_uri"
             android:nextFocusUp="@id/custom_uri"
             android:singleLine="true"
             android:textAppearance="?android:attr/textAppearanceMedium" />
 
         <CheckBox
             android:id="@+id/default_checkbox"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:layout_marginTop="16dp"
             android:layout_marginBottom="16dp"
             android:layout_marginStart="16dp"
             android:layout_marginEnd="16dp"
             android:text="@string/homepage_default_title" />
     </LinearLayout>
 
 </ScrollView>