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..cf52a30ee2fe43aee7e85e0a98aa5f5426ebb193 100644
--- a/content/public/browser/render_widget_host.h
+++ b/content/public/browser/render_widget_host.h
@@ -183,6 +183,10 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Sender {
   // source contents is used. |callback| is invoked with true on success, false
   // otherwise, along with a SkBitmap containing the copied pixel data.
   //
+  // If the backing store is not available or, in composited mode, the
+  // accelerated 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
   // asynchronously.