Implement a speedup for Y-only blurs by transposing.

src/opts/SkBlurImage_opts.h

 /*
  * Copyright 2013 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.
  */
 
 #ifndef SkBlurImage_opts_DEFINED
 #define SkBlurImage_opts_DEFINED
 
 #include "SkColorPriv.h"
 
 typedef void (*SkBoxBlurProc)(const SkPMColor* src, int srcStride, SkPMColor* dst, int kernelSize,
                               int leftOffset, int rightOffset, int width, int height);
 
 bool SkBoxBlurGetPlatformProcs(SkBoxBlurProc* boxBlurX,
                                SkBoxBlurProc* boxBlurY,
-                               SkBoxBlurProc* boxBlurXY);
+                               SkBoxBlurProc* boxBlurXY,
+                               SkBoxBlurProc* boxBlurYX);
 #endif