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

src/core/SkMatrix.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMatrix.h"
 #include "Sk64.h"
 #include "SkFloatBits.h"
@@ skipping 1159 matching lines @@
 SkScalar SkMatrix::mapRadius(SkScalar radius) const {
     SkVector    vec[2];
 
     vec[0].set(radius, 0);
     vec[1].set(0, radius);
     this->mapVectors(vec, 2);
 
     SkScalar d0 = vec[0].length();
     SkScalar d1 = vec[1].length();
 
-    return SkScalarMean(d0, d1);
+    // return geometric mean
+    return SkScalarSqrt(d0 * d1);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 void SkMatrix::Persp_xy(const SkMatrix& m, SkScalar sx, SkScalar sy,
                         SkPoint* pt) {
     SkASSERT(m.hasPerspective());
 
     SkScalar x = SkScalarMul(sx, m.fMat[kMScaleX]) +
                  SkScalarMul(sy, m.fMat[kMSkewX]) + m.fMat[kMTransX];
@@ skipping 802 matching lines @@
         rotation1->fX = cos1;
         rotation1->fY = sin1;
     }
     if (NULL != rotation2) {
         rotation2->fX = cos2;
         rotation2->fY = sin2;
     }
 
     return true;
 }