flag to use const& instead of const* for src-rect

BUG=skia:

Review URL: https://codereview.chromium.org/1272713005
diff --git a/tests/RecorderTest.cpp b/tests/RecorderTest.cpp
index 829fe3e..c41d634 100644
--- a/tests/RecorderTest.cpp
+++ b/tests/RecorderTest.cpp
@@ -98,7 +98,7 @@
         SkRecorder recorder(&record, 100, 100);
 
         // DrawImageRect is supposed to take a reference
-        recorder.drawImageRect(image.get(), 0, SkRect::MakeWH(100, 100));
+        recorder.drawImageRect(image.get(), SkRect::MakeWH(100, 100), nullptr);
         REPORTER_ASSERT(reporter, !image->unique());
 
         Tally tally;