Add SK_PREFETCH and use in SkBlurImageFilter.

src/effects/SkBlurImageFilter.cpp

Index: src/effects/SkBlurImageFilter.cpp
diff --git a/src/effects/SkBlurImageFilter.cpp b/src/effects/SkBlurImageFilter.cpp
index 2b823bc98b2b915db6086b87256c3e2e21940d4a..0fa54b555cd78cfae0b800b699a155f503bd5cd0 100644
--- a/src/effects/SkBlurImageFilter.cpp
+++ b/src/effects/SkBlurImageFilter.cpp
@@ -144,6 +144,8 @@ static void boxBlurY(const SkBitmap& src, SkBitmap* dst, int kernelSize,
                 sumB += SkGetPackedB32(r);
             }
             sptr += srcStride;
+            // The next leading pixel seems to be too hard to predict.  Hint the fetch.
+            SK_PREFETCH(sptr + (bottomOffset + 1) * srcStride);
             dptr += dstStride;
         }
     }