Use SkXfermode as public facing enum for GrPorterDuffXP

include/gpu/effects/GrCoverageSetOpXP.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrCoverageSetOpXP_DEFINED
 #define GrCoverageSetOpXP_DEFINED
 
 #include "GrTypes.h"
 #include "GrXferProcessor.h"
 #include "SkRegion.h"
 
 class GrProcOptInfo;
 
+/**
+ * This xfer processor directly blends the the src coverage with the dst using a set operator. It is
+ * useful for rendering coverage masks using CSG. It can optionally invert the src coverage before
+ * applying the set operator.
+ */
 class GrCoverageSetOpXPFactory : public GrXPFactory {
 public:
     static GrXPFactory* Create(SkRegion::Op regionOp, bool invertCoverage = false);
 
-    bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const SK_OVERRIDE {
+    bool supportsRGBCoverage(GrColor /*knownColor*/,
+                             uint32_t /*knownColorFlags*/) const SK_OVERRIDE {
         return true;
     }
 
-    bool canApplyCoverage(const GrProcOptInfo& colorPOI,
-                          const GrProcOptInfo& coveragePOI) const SK_OVERRIDE {
+    bool canApplyCoverage(const GrProcOptInfo& /*colorPOI*/,
+                          const GrProcOptInfo& /*coveragePOI*/) const SK_OVERRIDE {
         return true;
     }
 
     bool canTweakAlphaForCoverage() const SK_OVERRIDE { return false; }
 
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const SK_OVERRIDE;
 
 private:
     GrCoverageSetOpXPFactory(SkRegion::Op regionOp, bool invertCoverage);
 
     GrXferProcessor* onCreateXferProcessor(const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const SK_OVERRIDE;
 
-    bool willReadDstColor(const GrProcOptInfo& colorPOI,
-                          const GrProcOptInfo& coveragePOI) const SK_OVERRIDE {
+    bool willReadDstColor(const GrProcOptInfo& /*colorPOI*/,
+                          const GrProcOptInfo& /*coveragePOI*/) const SK_OVERRIDE {
         return false;
     }
 
     bool onIsEqual(const GrXPFactory& xpfBase) const SK_OVERRIDE {
         const GrCoverageSetOpXPFactory& xpf = xpfBase.cast<GrCoverageSetOpXPFactory>();
         return fRegionOp == xpf.fRegionOp;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
 
     SkRegion::Op fRegionOp;
     bool         fInvertCoverage;
 
     typedef GrXPFactory INHERITED;
 };
 #endif