add peekPixels to SkCanvas and SkSurface

gm/xfermodes3.cpp

Index: gm/xfermodes3.cpp
diff --git a/gm/xfermodes3.cpp b/gm/xfermodes3.cpp
index bb7d6149c5e3c790a2720b65b4b13597c1832bd2..76c4b88380d6c9d093a0b3522168572be1978a9b 100644
--- a/gm/xfermodes3.cpp
+++ b/gm/xfermodes3.cpp
@@ -168,8 +168,11 @@ private:
         if (NULL == layerCanvas) {
             canvas->restore();
         } else {
-            SkBitmap bitmap = layerCanvas->getDevice()->accessBitmap(false);
-            canvas->drawBitmap(bitmap, 0, 0);
+            SkAutoROCanvasPixels ropixels(layerCanvas);
+            SkBitmap bitmap;
+            if (ropixels.asROBitmap(&bitmap)) {
+                canvas->drawBitmap(bitmap, 0, 0);
+            }
         }
 
         r.inset(-SK_ScalarHalf, -SK_ScalarHalf);