Use SkXfermode as public facing enum for GrPorterDuffXP

BUG=skia:

Review URL: https://codereview.chromium.org/926593005
diff --git a/include/gpu/effects/GrCoverageSetOpXP.h b/include/gpu/effects/GrCoverageSetOpXP.h
new file mode 100644
index 0000000..e781999
--- /dev/null
+++ b/include/gpu/effects/GrCoverageSetOpXP.h
@@ -0,0 +1,66 @@
+/*
+ * 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 {
+        return true;
+    }
+
+    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 {
+        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
+