Adds kind-of-live thumbnail generation for a potential tab switcher.

chrome/browser/renderer_host/render_widget_host_painting_observer.h

+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_PAINTING_OBSERVER_H_
+#define CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_PAINTING_OBSERVER_H_
+
+class BackingStore;
+class RenderWidgetHost;
+
+// This class can be used to observe painting events for a RenderWidgetHost.
+// Its primary goal in Chrome is to allow thumbnails to be generated.
+class RenderWidgetHostPaintingObserver {
+ public:
+  // Indicates the RenderWidgetHost is about to destroy the backing store. The
+  // backing store will still be valid when this call is made.
+  virtual void WidgetWillDestroyBackingStore(RenderWidgetHost* widget,
+                                             BackingStore* backing_store) = 0;
+
+  // Indicates that the RenderWidgetHost just updated the backing store.
+  virtual void WidgetDidUpdateBackingStore(RenderWidgetHost* widget) = 0;
+};
+
+#endif  // CHROME_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_PAINTING_OBSERVER_H_