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/SurfaceSemaphoreTest.cpp b/tests/SurfaceSemaphoreTest.cpp
index b13c5d9..cd3b8e5 100644
--- a/tests/SurfaceSemaphoreTest.cpp
+++ b/tests/SurfaceSemaphoreTest.cpp
@@ -100,7 +100,7 @@
     // read pixels
     SkBitmap bitmap;
     bitmap.allocPixels(childII);
-    childCanvas->readPixels(bitmap, 0, 0);
+    childSurface->readPixels(bitmap, 0, 0);
 
     check_pixels(reporter, bitmap);
 }