Add NewRGB() with float gamma to SkColorSpace public API

include/core/SkColorSpace.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkColorSpace_DEFINED
 #define SkColorSpace_DEFINED
 
@@ skipping 71 matching lines @@
         /**
          *  Transfer function is the canonical sRGB curve, which has a short linear segment
          *  followed by a 2.4f exponential.
          */
         kSRGB_RenderTargetGamma,
     };
 
     /**
      *  Create an SkColorSpace from a transfer function and a color gamut.
      *
-     *  Transfer function can be specified as a render target or as the coefficients to an equation.
+     *  Transfer function can be specified as a render target, as the coefficients to an equation,
+     *  or as three exponents (R, G, B).
      *  Gamut is specified using the matrix transformation to XYZ D50.
      */
     static sk_sp<SkColorSpace> NewRGB(RenderTargetGamma gamma, const SkMatrix44& toXYZD50);
     static sk_sp<SkColorSpace> NewRGB(const SkColorSpaceTransferFn& coeffs,
                                       const SkMatrix44& toXYZD50);
+    static sk_sp<SkColorSpace> NewRGB(const float exponents[3], const SkMatrix44& toXYZD50);
 
     /**
      *  Create a common, named SkColorSpace.
      */
     static sk_sp<SkColorSpace> NewNamed(Named);
 
     /**
      *  Create an SkColorSpace from an ICC profile.
      */
     static sk_sp<SkColorSpace> NewICC(const void*, size_t);
@@ skipping 26 matching lines @@
      *  If both are null, we return true.  If one is null and the other is not, we return false.
      *  If both are non-null, we do a deeper compare.
      */
     static bool Equals(const SkColorSpace* src, const SkColorSpace* dst);
 
 protected:
     SkColorSpace() {}
 };
 
 #endif