Add matrix constructing helpers to SkMatrix

Review URL: https://codereview.chromium.org/1034273002
diff --git a/gm/texturedomaineffect.cpp b/gm/texturedomaineffect.cpp
index b85d208..6104d46 100644
--- a/gm/texturedomaineffect.cpp
+++ b/gm/texturedomaineffect.cpp
@@ -124,8 +124,7 @@
                     if (!fp) {
                         continue;
                     }
-                    SkMatrix viewMatrix;
-                    viewMatrix.setTranslate(x, y);
+                    const SkMatrix viewMatrix = SkMatrix::MakeTrans(x, y);
                     GrPipelineBuilder pipelineBuilder;
                     pipelineBuilder.setRenderTarget(rt);
                     pipelineBuilder.addColorProcessor(fp);