[Smart Lock, UI] Add ability to have an icon to the SnanackBar.

chrome/android/java/res/layout/snackbar.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. -->
 
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:minHeight="@dimen/snackbar_min_height"
     android:orientation="horizontal" >
 
+    <ImageView android:id="@+id/snackbar_icon"
+     android:layout_width="24dp"

[gone, 2015/11/10 00:35:04]

nit: indentation is messed up

+     android:layout_height="24dp"
+     android:layout_gravity="center_vertical"
+     android:layout_marginStart="12dp"

[gone, 2015/11/10 00:35:04]

can you explicitly state what ImageView.ScaleType to use in case a big icon is
passed in?  (Maybe FIT_CENTER?)

+     android:contentDescription="@null"/>
+
     <org.chromium.chrome.browser.snackbar.TemplatePreservingTextView
         android:id="@+id/snackbar_message"
         android:layout_width="0dp"
         android:layout_height="wrap_content"
         android:layout_gravity="start|center_vertical"
         android:layout_marginStart="24dp"
         android:layout_marginTop="14dp"
         android:layout_marginBottom="14dp"
         android:layout_weight="1"
         android:textAlignment="viewStart"
         android:textColor="@android:color/white"
         android:textSize="14sp" />
 
     <Button
         android:id="@+id/snackbar_button"
         android:layout_width="wrap_content"
         android:layout_height="@dimen/snackbar_min_height"
         android:layout_gravity="center_vertical"
         android:paddingEnd="24dp"
         android:paddingStart="24dp"
         android:fontFamily="sans-serif-medium"
         android:textAllCaps="true"
         android:textColor="@color/snackbar_button_color"
         android:textSize="14sp"
         style="@style/ButtonCompatBorderless" />
 
 </LinearLayout>