Fix up a few more lint warnings in chrome/

chrome/android/java/res/layout/preference_category.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. -->
 
 <!-- Emulates Android's preference_category_material.xml on pre-L devices.
      Also used as the layout for IconPreferenceCategory. -->
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:gravity="center_vertical"
     style="@style/PreferenceCategoryWithButtonStyle" >
 
     <TextView
         android:id="@android:id/title"
         android:layout_width="0dp"
         android:layout_weight="1"
         android:layout_height="wrap_content"
         android:textAppearance="@style/PreferenceCategoryTextStyle" />
 
+    <!-- Users of this layout are responsible to set contentBackground. -->

[newt (away), 2015/02/18 18:58:11]

contentDescription

[aurimas (slooooooooow), 2015/02/25 01:51:17]

Done

     <ImageView
         android:id="@android:id/icon"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:minHeight="@dimen/min_touch_target_size"
         android:minWidth="@dimen/min_touch_target_size"
         android:scaleType="center"
-        android:background="?android:attr/selectableItemBackground" />
+        android:background="?android:attr/selectableItemBackground"
+        android:contentDescription="@null" />
 
 </LinearLayout>