Implement SkBaseDevice snapshot support

include/core/SkDevice.h

Index: include/core/SkDevice.h
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 797850f88a069ad50bef84fcbce19b4f507356ca..6e02137fb5b817551f7ec2d74ed1fe60a3a888d6 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -17,6 +17,7 @@
 class SkClipStack;
 class SkDraw;
 class SkDrawFilter;
+class SkImage;
 struct SkIRect;
 class SkMatrix;
 class SkMetaData;
@@ -124,6 +125,13 @@ public:
 #endif
     };
 
+    virtual SkImage* newImageSnapshot() { return NULL; }
+
+    /** This makes the contents of this device undefined.
+     * See SkCanvas::discard()
+     */
+    virtual void discard() { }
+
 protected:
     enum TileUsage {
         kPossible_TileUsage,    //!< the created device may be drawn tiled
@@ -369,7 +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; }