Fix accessibility issues in custom tab UI

chrome/android/java/res/layout/custom_tabs_toolbar.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. -->
 
 <org.chromium.chrome.browser.toolbar.CustomTabToolbar xmlns:android="http://schemas.android.com/apk/res/android">
     <org.chromium.chrome.browser.widget.TintedImageButton
         android:id="@+id/back_button"
         style="@style/ToolbarButton"
         android:layout_gravity="center_vertical"
-        android:src="@drawable/btn_chevron_left" />
+        android:src="@drawable/btn_chevron_left"
+        android:contentDescription="@string/accessibility_toolbar_btn_back" />

[Yusuf, 2015/06/29 18:03:17]

is this the string we want to have? Or we need a new one?

[Ian Wen, 2015/06/29 18:27:42]

This string only says "Go back". Do you want me to create a string saying
something like "Go back to previous screen" or "Go back to client app" or "close
webpage"?

[Yusuf, 2015/06/29 18:30:07]

Yes, lets add a new one and say "Close webpage"

[Ian Wen, 2015/06/30 18:31:23]

Done.

     <ImageButton android:id="@+id/security_button"
         style="@style/LocationBarButton"
         android:layout_width="@dimen/location_bar_icon_width"
         android:layout_height="match_parent"
         android:layout_marginStart="48dp"
         android:scaleType="center"
         android:layout_gravity="center_vertical"
         android:alpha="0"
         android:visibility="gone"
         android:contentDescription="@string/accessibility_toolbar_btn_site_info" />
@@ skipping 48 matching lines @@
         android:contentDescription="@string/accessibility_toolbar_btn_menu"
         android:background="@null" />
     <org.chromium.chrome.browser.widget.ToolbarProgressBar
         android:id="@+id/progress"
         style="@style/Widget.AppCompat.ProgressBar.Horizontal"
         android:progressDrawable="@drawable/progress_bar"
         android:layout_width="match_parent"
         android:layout_height="2dp"
         android:progress="0" />
 </org.chromium.chrome.browser.toolbar.CustomTabToolbar>