Move canvas->surface association to the device subclasses

src/image/SkSurface_Base.h

Index: src/image/SkSurface_Base.h
diff --git a/src/image/SkSurface_Base.h b/src/image/SkSurface_Base.h
index 30af9761d362c4de4216981df6298cf48405bc9c..7e48bbf4c82c67cd9954b398245930f81e60ab10 100644
--- a/src/image/SkSurface_Base.h
+++ b/src/image/SkSurface_Base.h
@@ -66,23 +66,19 @@ public:
     // 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;
 }
@@ -90,7 +86,6 @@ SkCanvas* SkSurface_Base::getCachedCanvas() {
 SkImage* SkSurface_Base::getCachedImage(Budgeted budgeted) {
     if (NULL == fCachedImage) {
         fCachedImage = this->onNewImageSnapshot(budgeted);
-        SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     }
     return fCachedImage;
 }