Let client specify content description for custom action button

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"
     xmlns:chrome="http://schemas.android.com/apk/res-auto" >
     <ImageButton
         android:id="@+id/close_button"
@@ skipping 50 matching lines @@
         style="@style/ToolbarButton"
         android:layout_width="wrap_content"
         android:maxWidth="@dimen/custom_tabs_toolbar_maxWidth"
         android:paddingTop="@dimen/custom_tabs_toolbar_vertical_padding"
         android:paddingBottom="@dimen/custom_tabs_toolbar_vertical_padding"
         android:adjustViewBounds="true"
         android:scaleType="centerInside"
         android:layout_marginEnd="42dp"
         android:layout_gravity="center_vertical|end"
         android:visibility="gone"
-        android:contentDescription="@string/accessibility_toolbar_btn_custom" />
+        android:contentDescription="@null" />
     <org.chromium.chrome.browser.widget.TintedImageButton
         android:id="@+id/menu_button"
         style="@style/ToolbarButton"
         android:layout_gravity="center_vertical|end"
         android:layout_width="42dp"
         android:paddingEnd="4dp"
         android:src="@drawable/btn_menu"
         android:contentDescription="@string/accessibility_toolbar_btn_menu"
         android:background="@null" />
     <org.chromium.chrome.browser.widget.ToolbarProgressBar
         android:id="@+id/progress"
         android:layout_width="match_parent"
         android:layout_height="2dp"
         chrome:progressBarColor="@color/progress_bar_foreground"
         chrome:backgroundColor="@color/progress_bar_background" />
 </org.chromium.chrome.browser.toolbar.CustomTabToolbar>