Add a RWH_TabSwitchPaintDuration histogram.

chrome/browser/renderer_host/render_widget_host_view_gtk.h

Index: chrome/browser/renderer_host/render_widget_host_view_gtk.h
diff --git a/chrome/browser/renderer_host/render_widget_host_view_gtk.h b/chrome/browser/renderer_host/render_widget_host_view_gtk.h
index c2465811c5c6756f6e362d40c397de17c4bf7b10..94f4f1a16474cb174661602e1d77bd83d22b220f 100644
--- a/chrome/browser/renderer_host/render_widget_host_view_gtk.h
+++ b/chrome/browser/renderer_host/render_widget_host_view_gtk.h
@@ -120,6 +120,9 @@ class RenderWidgetHostViewGtk : public RenderWidgetHostView {
   // value returns true for is_null() if we are not recording whiteout times.
   base::TimeTicks whiteout_start_time_;
 
+  // The time it took after this view was selected for it to be fully painted.
+  base::TimeTicks tab_switch_paint_time_;
+
   // Variables used only for popups --------------------------------------------
   // Our parent widget.
   RenderWidgetHostView* parent_host_view_;