move SkScalarMean into its only caller, reducing out public API exposure

src/core/SkMatrix.cpp

Index: src/core/SkMatrix.cpp
diff --git a/src/core/SkMatrix.cpp b/src/core/SkMatrix.cpp
index 4493fababb3c11db23daf11de255e1010f959b5d..2c850b3fb94a708592181710a057d923730321e0 100644
--- a/src/core/SkMatrix.cpp
+++ b/src/core/SkMatrix.cpp
@@ -1177,7 +1177,8 @@ SkScalar SkMatrix::mapRadius(SkScalar radius) const {
     SkScalar d0 = vec[0].length();
     SkScalar d1 = vec[1].length();
 
-    return SkScalarMean(d0, d1);
+    // return geometric mean
+    return SkScalarSqrt(d0 * d1);
 }
 
 ///////////////////////////////////////////////////////////////////////////////