Add matrix constructing helpers to SkMatrix

Review URL: https://codereview.chromium.org/1034273002
diff --git a/gm/convexpolyeffect.cpp b/gm/convexpolyeffect.cpp
index 03f4634..294de55 100644
--- a/gm/convexpolyeffect.cpp
+++ b/gm/convexpolyeffect.cpp
@@ -191,9 +191,8 @@
                     SkDEBUGFAIL("Couldn't get Gr test target.");
                     return;
                 }
-                SkMatrix m;
+                const SkMatrix m = SkMatrix::MakeTrans(x, y);
                 SkPath p;
-                m.setTranslate(x, y);
                 path->transform(m, &p);
 
                 GrPrimitiveEdgeType edgeType = (GrPrimitiveEdgeType) et;