change offset to xy for pixelref subsetting

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/105893012

git-svn-id: http://skia.googlecode.com/svn/trunk@12958 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/core/SkBitmapDevice.h b/include/core/SkBitmapDevice.h
index f3d40d0..a9208e2 100644
--- a/include/core/SkBitmapDevice.h
+++ b/include/core/SkBitmapDevice.h
@@ -199,8 +199,8 @@
 
     SkPixelRef* getPixelRef() const { return fBitmap.pixelRef(); }
     // just for subclasses, to assign a custom pixelref
-    SkPixelRef* setPixelRef(SkPixelRef* pr, size_t offset) {
-        fBitmap.setPixelRef(pr, offset);
+    SkPixelRef* setPixelRef(SkPixelRef* pr) {
+        fBitmap.setPixelRef(pr);
         return pr;
     }