expose ConvertRadiusToSigma to aid clients in the API transition to sigma

NOTRY=True
R=robertphillips@google.com, fmalita@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/264923002

git-svn-id: http://skia.googlecode.com/svn/trunk@14501 2bbb7eff-a529-9590-31e7-b0007b416f81

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 */

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);