Implement SkBaseDevice snapshot support

include/core/SkSurface.h

Index: include/core/SkSurface.h
diff --git a/include/core/SkSurface.h b/include/core/SkSurface.h
index a87eccefdc9ee88796212ffa36072205538fe1e9..063475cec9c4f2e7cb6d410cb53ed338cd222095 100644
--- a/include/core/SkSurface.h
+++ b/include/core/SkSurface.h
@@ -109,37 +109,18 @@ public:
     int height() const { return fHeight; }
 
     /**
-     *  Returns a unique non-zero, unique value identifying the content of this
-     *  surface. Each time the content is changed changed, either by drawing
-     *  into this surface, or explicitly calling notifyContentChanged()) this
-     *  method will return a new value.
+     *  Returns a unique non-zero value identifying the content of this
+     *  surface. Each time the content is changed changed by drawing
+     *  into this surface through the canvas, this method will return a new
+     *  value.
      *
-     *  If this surface is empty (i.e. has a zero-dimention), this will return
+     *  Implementations guarantee that all generationIDs are unique among
+     *  all possible SkSurfaces and their possible contents.
+     *
+     *  If this surface is empty (i.e. has a zero-dimension), this will return
      *  0.
      */
-    uint32_t generationID();
-
-    /**
-     *  Modes that can be passed to notifyContentWillChange
-     */
-    enum ContentChangeMode {
-        /**
-         *  Use this mode if it is known that the upcoming content changes will
-         *  clear or overwrite prior contents, thus making them discardable.
-         */
-        kDiscard_ContentChangeMode,
-        /**
-         *  Use this mode if prior surface contents need to be preserved or
-         *  if in doubt.
-         */
-        kRetain_ContentChangeMode,
-    };
-
-    /**
-     *  Call this if the contents are about to change. This will (lazily) force a new
-     *  value to be returned from generationID() when it is called next.
-     */
-    void notifyContentWillChange(ContentChangeMode mode);
+    virtual uint32_t generationID() = 0;
 
     /**
      *  Return a canvas that will draw into this surface. This will always
@@ -221,16 +202,10 @@ protected:
     SkSurface(int width, int height, const SkSurfaceProps*);
     SkSurface(const SkImageInfo&, const SkSurfaceProps*);
 
-    // called by subclass if their contents have changed
-    void dirtyGenerationID() {
-        fGenerationID = 0;
-    }
-
 private:
     const SkSurfaceProps fProps;
     const int            fWidth;
     const int            fHeight;
-    uint32_t             fGenerationID;
 
     typedef SkRefCnt INHERITED;
 };