[Android Download] Show a progress bar before backend is loaded

chrome/android/java/res/layout/download_content.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. -->
 
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:background="#ffffff"
     android:orientation="vertical" >
 
     <org.chromium.chrome.browser.download.ui.DownloadManagerToolbar
         android:id="@+id/action_bar"
         android:layout_width="match_parent"
         android:layout_height="?attr/actionBarSize"
         android:background="@color/appbar_background"
         android:layout_alignParentTop="true" >
 
         <include layout="@layout/number_roll_view" />
 
     </org.chromium.chrome.browser.download.ui.DownloadManagerToolbar>
 
     <android.support.v7.widget.RecyclerView
         android:id="@+id/recycler_view"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:layout_alignParentBottom="true"
-        android:layout_below="@id/action_bar" />
+        android:layout_below="@id/action_bar"
+        android:visibility="gone" />
 
     <org.chromium.chrome.browser.widget.FadingShadowView
         android:id="@+id/shadow"
         android:layout_width="match_parent"
         android:layout_height="10dp"
         android:layout_below="@id/action_bar" />
 
     <LinearLayout
         android:id="@+id/empty_view"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerInParent="true"
         android:orientation="vertical"
         android:gravity="center"
         android:visibility="gone" >
 
         <ImageView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_marginBottom="3dp"
             app:srcCompat="@drawable/downloads_big" />
 
         <TextView
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:text="@string/download_manager_ui_empty"
             android:textColor="#5B5B5B"
             android:textSize="16sp" />
     </LinearLayout>
-</RelativeLayout>
+
+    <org.chromium.chrome.browser.widget.LoadingView
+        android:id="@+id/loading_view"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_centerInParent="true" />
+</RelativeLayout>