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 a61b5fa9acbb1529118b4e9d248a88003fdac636..13e8f1005b7005bd5e600421c308220b197766e1 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -332,6 +332,10 @@ bool RenderWidgetHostViewAura::HasFocus() const {
   return window_->HasFocus();
 }
 
+bool RenderWidgetHostViewAura::IsSurfaceAvailableForCopy() const {
+  return current_surface_ != 0;
+}
+
 void RenderWidgetHostViewAura::Show() {
   window_->Show();
 }
@@ -448,6 +452,8 @@ void RenderWidgetHostViewAura::CopyFromCompositingSurface(
   if (!compositor)
     return;
 
+  DCHECK(image_transport_clients_.find(current_surface_) !=
+         image_transport_clients_.end());

[mazda, 2012/05/23 20:06:23]

When this condition holds, |container| is initialized with NULL, so this
function returns as failure before calling GLHelper::CopyTextureTo, right?
I think just returning as failure is appropriate instead of causing CHECK error.

[mmocny, 2012/05/23 20:28:30]

You are right.  The issue was that we expect the existence of a client in the
map to imply !NULL, and so the use of map op[] was causing issues elsewhere. 
Changed this to exit early as you suggest instead.

On 2012/05/23 20:06:23, mazda wrote:

   ImageTransportClient* container = image_transport_clients_[current_surface_];
   if (!container)
     return;