Move canvas->surface association to the device subclasses

include/core/SkDevice.h

Index: include/core/SkDevice.h
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 9f3ac580080590c36989f4a657040e6a79092317..812425b2b747d4faf37213507a439add52e65271 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -123,6 +123,11 @@ public:
 #endif
     };
 
+    /** This makes the contents of this device undefined.
+     * See SkCanvas::discard()
+     */
+    virtual void discard() { }
+
 protected:
     enum Usage {
        kGeneral_Usage,
@@ -372,7 +377,7 @@ private:
     // but cannot change the width/height, so there should be no change to
     // any clip information.
     // TODO: move to SkBitmapDevice
-    virtual void replaceBitmapBackendForRasterSurface(const SkBitmap&) {}
+    virtual void replaceBitmapBackend(const SkBitmap&) {}
 
     virtual bool forceConservativeRasterClip() const { return false; }