Add matrix constructing helpers to SkMatrix

bench/DashBench.cpp

Index: bench/DashBench.cpp
diff --git a/bench/DashBench.cpp b/bench/DashBench.cpp
index 17a923492668b72e5924ba1a32f6738061d08e3a..bf0d45614308b6ff366e6cce4dff1b93442adffb 100644
--- a/bench/DashBench.cpp
+++ b/bench/DashBench.cpp
@@ -153,8 +153,7 @@ static void make_unit_star(SkPath* path, int n) {
 
 static void make_poly(SkPath* path) {
     make_unit_star(path, 9);
-    SkMatrix matrix;
-    matrix.setScale(SkIntToScalar(100), SkIntToScalar(100));
+    const SkMatrix matrix = SkMatrix::MakeScale(SkIntToScalar(100), SkIntToScalar(100));
     path->transform(matrix);
 }