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/SkImageTest.cpp b/tests/SkImageTest.cpp
index 4a05b44..426883b 100644
--- a/tests/SkImageTest.cpp
+++ b/tests/SkImageTest.cpp
@@ -37,11 +37,11 @@
 
     uint32_t pixel = 0;
     SkImageInfo info = SkImageInfo::Make(1, 1, kBGRA_8888_SkColorType, kUnpremul_SkAlphaType);
-    canvas.readPixels(info, &pixel, 4, 0, 0);
+    tgt.readPixels(info, &pixel, 4, 0, 0);
     REPORTER_ASSERT(reporter, pixel == SK_ColorGREEN);
-    canvas.readPixels(info, &pixel, 4, gWidth - 6, gWidth - 6);
+    tgt.readPixels(info, &pixel, 4, gWidth - 6, gWidth - 6);
     REPORTER_ASSERT(reporter, pixel == SK_ColorGREEN);
 
-    canvas.readPixels(info, &pixel, 4, gWidth - 5, gWidth - 5);
+    tgt.readPixels(info, &pixel, 4, gWidth - 5, gWidth - 5);
     REPORTER_ASSERT(reporter, pixel == SK_ColorTRANSPARENT);
 }