expose ConvertRadiusToSigma to aid clients in the API transition to sigma

include/effects/SkBlurMaskFilter.h

 /*
  * 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.
  */
 
 #ifndef SkBlurMaskFilter_DEFINED
 #define SkBlurMaskFilter_DEFINED
 
 // we include this since our callers will need to at least be able to ref/unref
 #include "SkMaskFilter.h"
 #include "SkScalar.h"
 #include "SkBlurTypes.h"
 
 class SK_API SkBlurMaskFilter {
 public:
 #ifdef SK_SUPPORT_LEGACY_BLURMASKFILTER_STYLE
     enum BlurStyle {
         kNormal_BlurStyle = kNormal_SkBlurStyle,  //!< fuzzy inside and outside
         kSolid_BlurStyle = kSolid_SkBlurStyle,   //!< solid inside, fuzzy outside
         kOuter_BlurStyle = kOuter_SkBlurStyle,   //!< nothing inside, fuzzy outside
         kInner_BlurStyle = kInner_SkBlurStyle,   //!< fuzzy inside, nothing outside
 
         kBlurStyleCount
     };
 #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 */
         kIgnoreTransform_BlurFlag   = 0x01,
         /** Use a smother, higher qulity blur algorithm */
         kHighQuality_BlurFlag       = 0x02,
         /** mask for all blur flags */
         kAll_BlurFlag = 0x03
     };
 
@@ skipping 36 matching lines @@
                                       SkScalar ambient, SkScalar specular,
                                       SkScalar blurRadius);
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 
 private:
     SkBlurMaskFilter(); // can't be instantiated
 };
 
 #endif