Use R.txt from AAR to generate R.java for it when building APK.

chrome/android/java/proguard.flags

 # 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.
 
 # Fragments are loaded using reflection via Fragment.instantiate(Context,String)
 # This can be improved upon - see crbug.com/622023.
 -keep public class com.google.android.apps.chrome** extends android.app.Fragment
 -keep public class org.chromium** extends android.app.Fragment
 
 # These classes aren't themselves referenced, but __ProcessService[0,1,2...] are
 # referenced, and we look up these services by appending a number onto the name
 # of the base class. Thus, we need to keep the base class name around so that
 # the child classes can be looked up.
 -keep class org.chromium.content.app.SandboxedProcessService
 -keep class org.chromium.content.app.PrivilegedProcessService
 
 # TODO(ianwen): remove this config once proguard file inclusion from AAR is supported.
 # CoordinatorLayout resolves the behaviors of its child components with reflection.
 -keep public class * extends android.support.design.widget.CoordinatorLayout$Behavior {
     public <init>(android.content.Context, android.util.AttributeSet);
     public <init>();
 }
 
 # SearchView is used in website_preferences_menu.xml and is constructed by
 # Android using reflection.
 -keep class android.support.v7.widget.SearchView {
   public <init>(...);
 }
 
-# Google Play Services warnings are about its resources.
--dontwarn com.google.android.gms.R**
-
 # The Google Play services library depends on the legacy Apache HTTP library,
 # and just adding it as proguard time dependency causes the following warnings:
 # `library class org.apache.http.params.HttpConnectionParams depends on program
 # class org.apache.http.params.HttpParams`. The library has its own
 # implementation of org.apache.http.params.HttpParams so it can safely be
 # ignored.
 -dontwarn org.apache.http.params.HttpParams