[Custom Tabs] Implement Bottombar

chrome/android/java/res/layout/main.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. -->
 
 <merge xmlns:android="http://schemas.android.com/apk/res/android">
     <LinearLayout
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:orientation="vertical">
 
         <org.chromium.chrome.browser.compositor.CompositorViewHolder
             android:id="@+id/compositor_view_holder"
             android:layout_width="match_parent"
             android:layout_height="0dp"
             android:layout_weight="1" />
 
         <HorizontalScrollView
             android:id="@+id/keyboard_accessory"
             android:layout_height="@dimen/keyboard_accessory_height"
             android:layout_width="match_parent"
             android:background="@drawable/keyboard_accessory_background"
             android:contentDescription="@string/autofill_keyboard_accessory_content_description"
             android:fillViewport="true"
             android:scrollbars="none"
             android:visibility="gone" />
+
+        <org.chromium.chrome.browser.widget.FadingShadowView
+            android:id="@+id/bottombar_shadow"
+            android:layout_width="match_parent"
+            android:layout_height="2dp"
+            android:layout_marginTop="-2dp"
+            android:visibility="gone" />
+
+        <ViewStub
+            android:id="@+id/bottombar_stub"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content" />
     </LinearLayout>
 
     <ViewStub
         android:id="@+id/omnibox_results_container_stub"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:layout_marginTop="@dimen/tab_strip_height"
         android:inflatedId="@+id/omnibox_results_container"
         android:layout="@layout/omnibox_results_container" />
 
     <!-- Toolbar size area painted black to avoid the action bar showing the window background -->
     <View
         android:id="@+id/action_bar_black_background"
         android:layout_width="match_parent"
         android:layout_height="@dimen/toolbar_height_no_shadow"
         android:background="#000000"
         android:visibility="gone" />
     <ViewStub
         android:id="@+id/control_container_stub"
         android:layout_width="match_parent"
         android:layout_height="wrap_content" />
     <ViewStub
         android:id="@+id/empty_container_stub"
         android:inflatedId="@+id/empty_container"
         android:layout="@layout/empty_background_view"
         android:layout_width="match_parent"
-        android:layout_height="match_parent"
-        />
+        android:layout_height="match_parent" />
     <!-- This empty view is used as the anchor for custom menu -->
     <View
         android:id="@+id/menu_anchor_stub"
         android:layout_width="0px"
         android:layout_height="0px"
         android:layout_gravity="bottom|start"
         />
 </merge>