change offset to xy for pixelref subsetting

src/image/SkSurface_Raster.cpp

Index: src/image/SkSurface_Raster.cpp
diff --git a/src/image/SkSurface_Raster.cpp b/src/image/SkSurface_Raster.cpp
index 61ade6f46c8ec6584c3a5a18995dff3a766cca2c..48afe4fc206a27d4e194460929985e6a12a34c2b 100644
--- a/src/image/SkSurface_Raster.cpp
+++ b/src/image/SkSurface_Raster.cpp
@@ -123,7 +123,7 @@ void SkSurface_Raster::onCopyOnWrite(ContentChangeMode mode) {
     if (SkBitmapImageGetPixelRef(this->getCachedImage()) == fBitmap.pixelRef()) {
         SkASSERT(fWeOwnThePixels);
         if (kDiscard_ContentChangeMode == mode) {
-            fBitmap.setPixelRef(NULL, 0);
+            fBitmap.setPixelRef(NULL);
             fBitmap.allocPixels();
         } else {
             SkBitmap prev(fBitmap);