Use SkCanvas::concat instead of SkCanvas::setMatrix so this GM can be tiled.

Review URL: https://codereview.appspot.com/6460091

git-svn-id: http://skia.googlecode.com/svn/trunk@5116 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/gradients.cpp b/gm/gradients.cpp
index 9336052..775c796 100644
--- a/gm/gradients.cpp
+++ b/gm/gradients.cpp
@@ -204,7 +204,7 @@
         perspective.setIdentity();
         perspective.setPerspY(SkScalarDiv(SK_Scalar1, SkIntToScalar(1000)));
         perspective.setSkewX(SkScalarDiv(SkIntToScalar(8), SkIntToScalar(25)));
-        canvas->setMatrix(perspective);
+        canvas->concat(perspective);
         INHERITED::onDraw(canvas);
     }