Don't drop all children when changing Android root layer.

content/browser/renderer_host/compositor_impl_android.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_BROWSER_RENDERER_HOST_COMPOSITOR_IMPL_ANDROID_H_
 #define CONTENT_BROWSER_RENDERER_HOST_COMPOSITOR_IMPL_ANDROID_H_
 
 #include "base/basictypes.h"
 #include "base/cancelable_callback.h"
 #include "base/compiler_specific.h"
@@ skipping 114 matching lines @@
       current_composite_task_->Cancel();
     current_composite_task_.reset();
     composite_on_vsync_trigger_ = DO_NOT_COMPOSITE;
     will_composite_immediately_ = false;
   }
   cc::UIResourceId GenerateUIResourceFromUIResourceBitmap(
       const cc::UIResourceBitmap& bitmap,
       bool is_transient);
   void OnGpuChannelEstablished();
 
+  // root_layer_ is the persistent internal root layer, while subroot_layer_
+  // is the one attached by the compositor client.
   scoped_refptr<cc::Layer> root_layer_;
+  scoped_refptr<cc::Layer> subroot_layer_;
+
   scoped_ptr<cc::LayerTreeHost> host_;
   content::UIResourceProviderImpl ui_resource_provider_;
 
   gfx::Size size_;
   bool has_transparent_background_;
   float device_scale_factor_;
 
   ANativeWindow* window_;
   int surface_id_;
 
@@ skipping 32 matching lines @@
   base::TimeTicks last_vsync_;
 
   base::WeakPtrFactory<CompositorImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(CompositorImpl);
 };
 
 } // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_COMPOSITOR_IMPL_ANDROID_H_