make setScaleTranslate public

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2122093002

TBR=

Review-Url: https://codereview.chromium.org/2122093002
diff --git a/tests/MatrixTest.cpp b/tests/MatrixTest.cpp
index 01b2ba4..c4eb9c7 100644
--- a/tests/MatrixTest.cpp
+++ b/tests/MatrixTest.cpp
@@ -939,6 +939,11 @@
     test_set9(reporter);
 
     test_decompScale(reporter);
+
+    mat.setScaleTranslate(2, 3, 1, 4);
+    mat2.setScale(2, 3);
+    mat2.postTranslate(1, 4);
+    REPORTER_ASSERT(reporter, mat == mat2);
 }
 
 DEF_TEST(Matrix_Concat, r) {