Implement support for KHR_blend_equation_advanced

src/gpu/effects/GrCustomXfermodePriv.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrCustomXfermodePriv_DEFINED
 #define GrCustomXfermodePriv_DEFINED
 
@@ skipping 57 matching lines @@
                             GrXPFactory::InvariantOutput*) const override;
 
 private:
     GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
                                            const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const override; 
 
     bool willReadDstColor(const GrDrawTargetCaps& caps,
                           const GrProcOptInfo& colorPOI,
-                          const GrProcOptInfo& coveragePOI) const override {
-        return true;
-    }
+                          const GrProcOptInfo& coveragePOI) const override;
 
     bool onIsEqual(const GrXPFactory& xpfBase) const override {
         const GrCustomXPFactory& xpf = xpfBase.cast<GrCustomXPFactory>();
         return fMode == xpf.fMode;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     SkXfermode::Mode fMode;
 
     typedef GrXPFactory INHERITED;
 };
 #endif