Prevent browser thumbnailer from trying to read from frontbuffer (surface texture) when none exists.

content/browser/renderer_host/render_widget_host_view_aura.cc

Index: content/browser/renderer_host/render_widget_host_view_aura.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_aura.cc b/content/browser/renderer_host/render_widget_host_view_aura.cc
index 5bb4f3b0d0b60a037fe4a0b6729995cff73a49a7..4c27d8f7872a16bfb4162c4ed2866a5ea8a5fa44 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -333,6 +333,10 @@ bool RenderWidgetHostViewAura::HasFocus() const {
   return window_->HasFocus();
 }
 
+bool RenderWidgetHostViewAura::IsSurfaceAvailableForCopy() const {
+  return current_surface_ != 0;
+}
+
 void RenderWidgetHostViewAura::Show() {
   window_->Show();
 }
@@ -449,10 +453,14 @@ void RenderWidgetHostViewAura::CopyFromCompositingSurface(
   if (!compositor)
     return;
 
-  ImageTransportClient* container = image_transport_clients_[current_surface_];
-  if (!container)
+  std::map<uint64, scoped_refptr<ImageTransportClient> >::iterator it =
+      image_transport_clients_.find(current_surface_);
+  if (it == image_transport_clients_.end())
     return;
 
+  ImageTransportClient* container = it->second;
+  DCHECK(container);
+
   gfx::Size size_in_pixel = content::ConvertSizeToPixel(this, size);
   if (!output->initialize(
       size_in_pixel.width(), size_in_pixel.height(), true))