Support browser side thumbnailing for GPU composited pages on Windows.

content/public/browser/render_widget_host.h

Index: content/public/browser/render_widget_host.h
diff --git a/content/public/browser/render_widget_host.h b/content/public/browser/render_widget_host.h
index 72846b73fba46549764bd2c2adc990842589b263..bad57462e31cf3b9c9cfe7fcfd1076880373d2c7 100644
--- a/content/public/browser/render_widget_host.h
+++ b/content/public/browser/render_widget_host.h
@@ -190,6 +190,12 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Channel::Sender {
                                                CGContextRef target) = 0;
 #endif
 
+  // Copies the contents of the compositing surface into the given
+  // (uninitialized) PlatformCanvas if any. Returns true on success, false
+  // otherwise.
+  virtual bool CopyFromCompositingSurface(const gfx::Size& size,
+                                          skia::PlatformCanvas* output) = 0;
+
   // Enable renderer accessibility. This should only be called when a
   // screenreader is detected.
   virtual void EnableRendererAccessibility() = 0;