don't rely on canvas->readPixels

Bug: skia:3216
Change-Id: I01a8d0083c79c0fe71fbc4d8cfdde4fa6e48b636
Reviewed-on: https://skia-review.googlesource.com/25741
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/tests/ReadPixelsTest.cpp b/tests/ReadPixelsTest.cpp
index f426e5f..ce01004 100644
--- a/tests/ReadPixelsTest.cpp
+++ b/tests/ReadPixelsTest.cpp
@@ -359,7 +359,7 @@
                     fill_dst_bmp_with_init_data(&bmp);
                 }
                 uint32_t idBefore = surface->generationID();
-                bool success = canvas->readPixels(bmp, srcRect.fLeft, srcRect.fTop);
+                bool success = surface->readPixels(bmp, srcRect.fLeft, srcRect.fTop);
                 uint32_t idAfter = surface->generationID();
 
                 // we expect to succeed when the read isn't fully clipped