Allow CopyFromBackingStore to fallback to copying from the renderer side if the accelerated surface…

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 113477a4a2cf000f57e63292b1ccac7c476dea85..52a2ff7ab4be110020d784394482de602cfdee99 100644
--- a/content/public/browser/render_widget_host.h
+++ b/content/public/browser/render_widget_host.h
@@ -181,7 +181,9 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Sender {
   // |accelerated_dst_size| is larger than the content size, the content is not
   // resized. If |accelerated_dst_size| is empty, the size copied from the
   // source contents is used. |callback| is invoked with true on success, false
-  // otherwise, along with a SkBitmap containing the copied pixel data.
+  // otherwise, along with a SkBitmap containing the copied pixel data. If
+  // accelerated compositing is active, but the surface is not available to the
+  // browser, then the copy is done on the renderer side via IPC.
   //
   // NOTE: |callback| is called synchronously if the backing store is available.
   // When accelerated compositing is active, |callback| may be called