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 6850ebd13809f70a7a33f34ca13f2c8b5e943611..cf32b888b96f369914faf8043ea8d864df5595a5 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -329,6 +329,10 @@ bool RenderWidgetHostViewAura::HasFocus() const {
   return window_->HasFocus();
 }
 
+bool RenderWidgetHostViewAura::HasSurface() const {
+  return current_surface_ != 0;
+}
+
 void RenderWidgetHostViewAura::Show() {
   window_->Show();
 }