expose ConvertRadiusToSigma to aid clients in the API transition to sigma

include/effects/SkBlurMaskFilter.h

Index: include/effects/SkBlurMaskFilter.h
diff --git a/include/effects/SkBlurMaskFilter.h b/include/effects/SkBlurMaskFilter.h
index ddb163e58ab6667cc525b7b60718d35f5218e093..5fcf463e982e6072c64f2dd308527e10055b1bfc 100644
--- a/include/effects/SkBlurMaskFilter.h
+++ b/include/effects/SkBlurMaskFilter.h
@@ -26,6 +26,12 @@ public:
     };
 #endif
 
+    /**
+     *  If radius > 0, return the corresponding sigma, else return 0. Use this to convert from the
+     *  (legacy) idea of specify the blur "radius" to the standard notion of specifying its sigma.
+     */
+    static SkScalar ConvertRadiusToSigma(SkScalar radius);
+
     enum BlurFlags {
         kNone_BlurFlag = 0x00,
         /** The blur layer's radius is not affected by transforms */