Move canvas->surface association to the device subclasses

src/image/SkSurface_Base.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSurface_Base_DEFINED
 #define SkSurface_Base_DEFINED
 
@@ skipping 48 matching lines @@
      *  being shared with the cachedImage.
      */
     virtual void onCopyOnWrite(ContentChangeMode) = 0;
 
     inline SkCanvas* getCachedCanvas();
     inline SkImage* getCachedImage(Budgeted);
 
     // called by SkSurface to compute a new genID
     uint32_t newGenerationID();
 
+    void aboutToDraw(ContentChangeMode mode);
+
 private:
     SkCanvas*   fCachedCanvas;
     SkImage*    fCachedImage;
 
-    void aboutToDraw(ContentChangeMode mode);
-    friend class SkCanvas;
     friend class SkSurface;
-
     typedef SkSurface INHERITED;
 };
 
 SkCanvas* SkSurface_Base::getCachedCanvas() {
     if (NULL == fCachedCanvas) {
         fCachedCanvas = this->onNewCanvas();
-        if (fCachedCanvas) {
-            fCachedCanvas->setSurfaceBase(this);
-        }
     }
     return fCachedCanvas;
 }
 
 SkImage* SkSurface_Base::getCachedImage(Budgeted budgeted) {
     if (NULL == fCachedImage) {
         fCachedImage = this->onNewImageSnapshot(budgeted);
-        SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     }
     return fCachedImage;
 }
 
 #endif