Add matrix constructing helpers to SkMatrix

Review URL: https://codereview.chromium.org/1034273002
diff --git a/bench/PathBench.cpp b/bench/PathBench.cpp
index a429e2f..710179a 100644
--- a/bench/PathBench.cpp
+++ b/bench/PathBench.cpp
@@ -57,8 +57,7 @@
         SkPath path;
         this->makePath(&path);
         if (fFlags & kBig_Flag) {
-            SkMatrix m;
-            m.setScale(SkIntToScalar(10), SkIntToScalar(10));
+            const SkMatrix m = SkMatrix::MakeScale(SkIntToScalar(10), SkIntToScalar(10));
             path.transform(m);
         }