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/ResourceCacheTest.cpp b/tests/ResourceCacheTest.cpp
index 3cfa157..b018b8f 100644
--- a/tests/ResourceCacheTest.cpp
+++ b/tests/ResourceCacheTest.cpp
@@ -67,7 +67,7 @@
 
     for (int i = 0; i < 100; ++i) {
         canvas->drawBitmap(src, 0, 0);
-        canvas->readPixels(readback, 0, 0);
+        surface->readPixels(readback, 0, 0);
 
         // "modify" the src texture
         src.notifyPixelsChanged();