Create a GraphicsContext from a DisplayList.

Source/platform/graphics/GraphicsContext.h

 /*
  * Copyright (C) 2003, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008-2009 Torch Mobile, Inc.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 57 matching lines @@
     enum AccessMode {
         ReadOnly,
         ReadWrite
     };
 
     enum DisabledMode {
         NothingDisabled = 0, // Run as normal.
         FullyDisabled = 1 // Do absolutely minimal work to remove the cost of the context from performance tests.
     };
 
+    // A 0 canvas is allowed, but in such cases the context must only have canvas
+    // related commands called when within a beginRecording/endRecording block.
+    // Furthermore, save/restore calls must be balanced any time the canvas is 0.
     explicit GraphicsContext(SkCanvas*, DisabledMode = NothingDisabled);
+
     ~GraphicsContext();
 
     // Returns the canvas used for painting. Must not be called if painting is disabled.
     // Accessing the backing canvas this way flushes all queued save ops,
     // so it should be avoided. Use the corresponding draw/matrix/clip methods instead.
     SkCanvas* canvas()
     {
         // Flush any pending saves.
         realizeCanvasSave();
 
@@ skipping 384 matching lines @@
             }
         }
     }
 
     // Apply deferred canvas state saves
     void realizeCanvasSave()
     {
         if (!m_pendingCanvasSave || contextDisabled())
             return;
 
+        ASSERT(m_canvas); // m_pendingCanvasSave should never be true when no canvas.
         m_canvas->save();
         m_pendingCanvasSave = false;
     }
 
     void didDrawTextInRect(const SkRect& textRect);
 
     void fillRectWithRoundedHole(const IntRect&, const RoundedRect& roundedHoleRect, const Color&);
 
     bool isRecording() const;
 
@@ skipping 43 matching lines @@
     bool m_accelerated : 1;
     bool m_isCertainlyOpaque : 1;
     bool m_printing : 1;
     bool m_antialiasHairlineImages : 1;
     bool m_shouldSmoothFonts : 1;
 };
 
 } // namespace blink
 
 #endif // GraphicsContext_h