Override SkCanvas::drawImage() in SkRecorder.

BUG=skia:2947

Review URL: https://codereview.chromium.org/610003002
diff --git a/src/image/SkImage_Raster.cpp b/src/image/SkImage_Raster.cpp
index d13141c..18d6a8c 100644
--- a/src/image/SkImage_Raster.cpp
+++ b/src/image/SkImage_Raster.cpp
@@ -217,8 +217,8 @@
     return SkNEW_ARGS(SkImage_Raster, (info, pr, rowBytes));
 }
 
-SkPixelRef* SkBitmapImageGetPixelRef(SkImage* image) {
-    return ((SkImage_Raster*)image)->getPixelRef();
+const SkPixelRef* SkBitmapImageGetPixelRef(const SkImage* image) {
+    return ((const SkImage_Raster*)image)->getPixelRef();
 }
 
 bool SkImage_Raster::isOpaque() const {