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

Unlike when this method was originally written, there can now be a
second child, a readback layer.  If the root layer happens to change
while a readback is active, it will never complete, hanging the
rendering.

NOTRY=true
BUG=386337

Review URL: https://codereview.chromium.org/407433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284164 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/renderer_host/compositor_impl_android.cc

@@ -341,9 +341,14 @@ UIResourceProvider& CompositorImpl::GetUIResourceProvider() {
 }
 
 void CompositorImpl::SetRootLayer(scoped_refptr<cc::Layer> root_layer) {
-  root_layer_->RemoveAllChildren();
-  if (root_layer)
+  if (subroot_layer_) {
+    subroot_layer_->RemoveFromParent();
+    subroot_layer_ = NULL;
+  }
+  if (root_layer) {
+    subroot_layer_ = root_layer;
     root_layer_->AddChild(root_layer);
+  }
 }
 
 void CompositorImpl::SetWindowSurface(ANativeWindow* window) {

content/browser/renderer_host/compositor_impl_android.h

@@ -132,7 +132,11 @@ class CONTENT_EXPORT CompositorImpl
       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_;