expose ConvertRadiusToSigma to aid clients in the API transition to sigma

src/effects/SkBlurMaskFilter.cpp

Index: src/effects/SkBlurMaskFilter.cpp
diff --git a/src/effects/SkBlurMaskFilter.cpp b/src/effects/SkBlurMaskFilter.cpp
index 5dffd6fd1f1d571eb6b2fac407eebd6cba8a5684..2169a42cb6df9e043b34209de947f3d101d908a3 100644
--- a/src/effects/SkBlurMaskFilter.cpp
+++ b/src/effects/SkBlurMaskFilter.cpp
@@ -28,6 +28,10 @@
 #include "SkDraw.h"
 #endif
 
+SkScalar SkBlurMaskFilter::ConvertRadiusToSigma(SkScalar radius) {
+    return SkBlurMask::ConvertRadiusToSigma(radius);
+}
+
 class SkBlurMaskFilterImpl : public SkMaskFilter {
 public:
     SkBlurMaskFilterImpl(SkScalar sigma, SkBlurStyle, uint32_t flags);