Add matrix constructing helpers to SkMatrix

gm/multipicturedraw.cpp

Index: gm/multipicturedraw.cpp
diff --git a/gm/multipicturedraw.cpp b/gm/multipicturedraw.cpp
index 0625b5a35bcb9558e77717505651abdcedbf361f..f31b6985660b8f1bce4e4eb3c529b0aa8995e082 100644
--- a/gm/multipicturedraw.cpp
+++ b/gm/multipicturedraw.cpp
@@ -423,8 +423,8 @@ static void tiled(SkCanvas* finalCanvas, SkMultiPictureDraw* mpd,
 
             SkCanvas* subCanvas = step.fSurf->getCanvas();
 
-            SkMatrix trans;
-            trans.setTranslate(-SkIntToScalar(x*kTileWidth), -SkIntToScalar(y*kTileHeight));
+            const SkMatrix trans = SkMatrix::MakeTrans(-SkIntToScalar(x*kTileWidth), 
+                                                       -SkIntToScalar(y*kTileHeight));
 
             create_content(mpd, pfGen, pictures, subCanvas, trans);
         }