add SkDrawPictureCallback optional parameter to drawPicture(), which can abort the picture drawing.

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 2bf5a54a51ae298d0947c063eb41882f1cb7d0a6..18abbdcf41d3e5081bf001fe3c169dc91078d434 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -30,6 +30,24 @@ class SkPicture;
 class SkRRect;
 class SkSurface_Base;
 
+/**
+ *  Subclasses of this can be passed to canvas.drawPicture. During the drawing
+ *  of the picture, this callback will periodically be invoked. If its
+ *  abortDrawing() returns true, then picture playback will be interrupted.
+ *
+ *  The resulting drawing is undefined, as there is no guarantee how often the
+ *  callback will be invoked. If the abort happens inside some level of nested
+ *  calls to save(), restore will automatically be called to return the state
+ *  to the same level it was before the drawPicture call was made.
+ */
+class SkDrawPictureCallback {
+public:
+    SkDrawPictureCallback() {}
+    virtual ~SkDrawPictureCallback() {}
+
+    virtual bool abortDrawing() = 0;
+};
+
 /** \class SkCanvas
 
     A Canvas encapsulates all of the state about drawing into a device (bitmap).
@@ -808,13 +826,14 @@ public:
                            const SkPath& path, const SkMatrix* matrix);
 #endif
 
+    
     /** Draw the picture into this canvas. This method effective brackets the
         playback of the picture's draw calls with save/restore, so the state
         of this canvas will be unchanged after this call.
         @param picture The recorded drawing commands to playback into this
                        canvas.
     */
-    virtual void drawPicture(SkPicture& picture);
+    virtual void drawPicture(SkPicture& picture, SkDrawPictureCallback* = NULL);
 
     enum VertexMode {
         kTriangles_VertexMode,