Implement SkBaseDevice snapshot support

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index b44e90975736285c30d594715f76257d9ff1b3cf..f01042ed298a4e8c9832a15a73dac059c776682b 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -29,7 +29,6 @@ class SkMetaData;
 class SkPicture;
 class SkRRect;
 class SkSurface;
-class SkSurface_Base;
 class SkTextBlob;
 class GrContext;
 class GrRenderTarget;
@@ -598,14 +597,14 @@ public:
     }
 
     /**
-     * This makes the contents of the canvas undefined. Subsequent calls that
-     * require reading the canvas contents will produce undefined results. Examples
-     * include blending and readPixels. The actual implementation is backend-
-     * dependent and one legal implementation is to do nothing. Like clear(), this
-     * ignores the clip.
+     * This makes the contents of current layer of the canvas
+     * undefined. Subsequent calls that require reading the canvas contents will
+     * produce undefined results. Examples include blending and readPixels. The
+     * actual implementation is backend- dependent and one legal implementation
+     * is to do nothing. Like clear(), this ignores the clip.
      *
      * This function should only be called if the caller intends to subsequently
-     * draw to the canvas. The canvas may do real work at discard() time in order
+     * draw to the current layer. The canvas may do real work at discard() time in order
      * to optimize performance on subsequent draws. Thus, if you call this and then
      * never draw to the canvas subsequently you may pay a perfomance penalty.
      */
@@ -1258,10 +1257,6 @@ protected:
                         SkIRect* intersection,
                         const SkImageFilter* imageFilter = NULL);
 
-    // notify our surface (if we have one) that we are about to draw, so it
-    // can perform copy-on-write or invalidate any cached images
-    void predrawNotify();
-
 private:
     class MCRec;
 
@@ -1278,11 +1273,6 @@ private:
 
     SkMetaData* fMetaData;
 
-    SkSurface_Base*  fSurfaceBase;
-    SkSurface_Base* getSurfaceBase() const { return fSurfaceBase; }
-    void setSurfaceBase(SkSurface_Base* sb) {
-        fSurfaceBase = sb;
-    }
     friend class SkSurface_Base;
     friend class SkSurface_Gpu;