Implement SkBaseDevice snapshot support

include/core/SkBitmapDevice.h

Index: include/core/SkBitmapDevice.h
diff --git a/include/core/SkBitmapDevice.h b/include/core/SkBitmapDevice.h
index a35f9df412cd86c6302c772eb4269385367fd231..30abd32601988ae5694159ba45e17d16689e5b77 100644
--- a/include/core/SkBitmapDevice.h
+++ b/include/core/SkBitmapDevice.h
@@ -34,9 +34,11 @@ public:
     static SkBitmapDevice* Create(const SkImageInfo& info) {
         return Create(info, NULL);
     }
-
+    ~SkBitmapDevice();
     SkImageInfo imageInfo() const SK_OVERRIDE;
 
+    SkImage* newImageSnapshot() SK_OVERRIDE;
+    void discard() SK_OVERRIDE;
 protected:
     bool onShouldDisableLCD(const SkPaint&) const SK_OVERRIDE;
 
@@ -137,13 +139,15 @@ private:
     friend class SkDeviceImageFilterProxy;
 
     friend class SkSurface_Raster;
+    friend class SkRasterDevice;
 
     // used to change the backend's pixels (and possibly config/rowbytes)
     // but cannot change the width/height, so there should be no change to
     // any clip information.
-    void replaceBitmapBackendForRasterSurface(const SkBitmap&) SK_OVERRIDE;
+    void deepCopyBackendIfNeeded();
 
     SkBaseDevice* onCreateDevice(const CreateInfo&, const SkPaint*) SK_OVERRIDE;
+    virtual SkImage* onNewImageSnapshot();
 
     SkSurface* newSurface(const SkImageInfo&, const SkSurfaceProps&) SK_OVERRIDE;
     const void* peekPixels(SkImageInfo*, size_t* rowBytes) SK_OVERRIDE;
@@ -151,7 +155,7 @@ private:
     SkImageFilter::Cache* getImageFilterCache() SK_OVERRIDE;
 
     SkBitmap    fBitmap;
-
+    SkAutoTUnref<SkImage> fSnapshot;
     typedef SkBaseDevice INHERITED;
 };