canvas: make a temporary buffer when a context doesn't exist.

Source/core/editing/Editor.cpp

Index: Source/core/editing/Editor.cpp
diff --git a/Source/core/editing/Editor.cpp b/Source/core/editing/Editor.cpp
index d317fc8515b7664a67605f732448e845cccd1797..f19039bc7a66816b9b262a91be455c7af3027087 100644
--- a/Source/core/editing/Editor.cpp
+++ b/Source/core/editing/Editor.cpp
@@ -421,7 +421,7 @@ void Editor::writeSelectionToPasteboard(Pasteboard* pasteboard, Range* selectedR
     pasteboard->writeHTML(html, url, plainText, canSmartCopyOrDelete());
 }
 
-static Image* imageFromNode(const Node& node)
+static PassRefPtr<Image> imageFromNode(const Node& node)
 {
     node.document().updateLayoutIgnorePendingStylesheets();
     RenderObject* renderer = node.renderer();