🏠 Add expand button and flag to enable it

chrome/android/java/res/layout/toolbar_phone_common.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2016 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. -->
 
 <!-- The toolbar containing the URL bar, back button, and NTP button.
 -->
 <merge xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:chrome="http://schemas.android.com/apk/res-auto">
 
     <org.chromium.chrome.browser.widget.newtab.NewTabButton
         android:id="@+id/new_tab_button"
         style="@style/ToolbarButton"
         android:layout_width="wrap_content"
         android:layout_gravity="start|top"
         android:paddingStart="16dp"
         android:paddingEnd="16dp"
         android:visibility="invisible"
         android:background="?attr/selectableItemBackground"
         android:contentDescription="@string/accessibility_toolbar_btn_new_tab" />
 
     <org.chromium.chrome.browser.toolbar.HomePageButton
         android:id="@+id/home_button"
         style="@style/ToolbarButton"
         android:src="@drawable/btn_toolbar_home"
         android:contentDescription="@string/accessibility_toolbar_btn_home"
         android:visibility="gone" />
 
+    <org.chromium.chrome.browser.widget.TintedImageButton
+        android:id="@+id/expand_sheet_button"
+        style="@style/ToolbarButton"
+        android:src="@drawable/ic_collapsed"
+        android:contentDescription="@string/accessibility_toolbar_btn_expand"
+        android:visibility="gone" />
+
     <org.chromium.chrome.browser.omnibox.LocationBarPhone
         android:id="@+id/location_bar"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:layout_gravity="top" />
 
     <LinearLayout android:id="@+id/toolbar_buttons"
         android:orientation="horizontal"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
@@ skipping 23 matching lines @@
                 android:id="@+id/menu_badge"
                 style="@style/ToolbarMenuButtonPhone"
                 android:src="@drawable/badge_update_dark"
                 android:contentDescription="@null"
                 android:importantForAccessibility="no"
                 android:visibility="invisible" />
         </FrameLayout>
     </LinearLayout>
 
 </merge>