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

content/browser/renderer_host/test_render_view_host.cc

Index: content/browser/renderer_host/test_render_view_host.cc
diff --git a/content/browser/renderer_host/test_render_view_host.cc b/content/browser/renderer_host/test_render_view_host.cc
index e086b7f14a40bb4fdc9ea8b959801e15be042857..56286875f4333298664126c8aa6ca7b9ccfba008 100644
--- a/content/browser/renderer_host/test_render_view_host.cc
+++ b/content/browser/renderer_host/test_render_view_host.cc
@@ -69,6 +69,10 @@ bool TestRenderWidgetHostView::HasFocus() const {
   return true;
 }
 
+bool TestRenderWidgetHostView::IsSurfaceAvailableForCopy() const {
+  return true;
+}
+
 void TestRenderWidgetHostView::Show() {
   is_showing_ = true;
 }