Simplify common case for recttorect

Change-Id: I70e295a677b8cac3d578e3cd57472c833af03877
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/354336
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/tools/viewer/Viewer.cpp b/tools/viewer/Viewer.cpp
index 1b4e096..9e80623 100644
--- a/tools/viewer/Viewer.cpp
+++ b/tools/viewer/Viewer.cpp
@@ -1134,7 +1134,8 @@
         // Start with a matrix that scales the slide to the available screen space
         if (fWindow->scaleContentToFit()) {
             if (windowRect.width() > 0 && windowRect.height() > 0) {
-                fDefaultMatrix.setRectToRect(slideBounds, windowRect, SkMatrix::kStart_ScaleToFit);
+                fDefaultMatrix = SkMatrix::RectToRect(slideBounds, windowRect,
+                                                      SkMatrix::kStart_ScaleToFit);
             }
         }