Don't create a GXPFactory when blend is SrcOver

include/gpu/GrXferProcessor.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 GrXferProcessor_DEFINED
 #define GrXferProcessor_DEFINED
 
@@ skipping 298 matching lines @@
  * blend with the destination color.
  */
 class GrXPFactory : public SkRefCnt {
 public:
     typedef GrXferProcessor::DstTexture DstTexture;
     GrXferProcessor* createXferProcessor(const GrProcOptInfo& colorPOI,
                                          const GrProcOptInfo& coveragePOI,
                                          bool hasMixedSamples,
                                          const DstTexture*,
                                          const GrCaps& caps) const;
-
-    /**
-     * This function returns true if the GrXferProcessor generated from this factory will be able to
-     * correctly blend when using RGB coverage. The knownColor and knownColorFlags represent the
-     * final computed color from the color stages.
-     */
-    virtual bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const = 0;
-
     /**
      * Known color information after blending, but before accounting for any coverage.
      */
     struct InvariantBlendedColor {
         bool                     fWillBlendWithDst;
         GrColor                  fKnownColor;
         GrColorComponentFlags    fKnownColorFlags;
     };
 
     /** 
@@ skipping 64 matching lines @@
     enum {
         kIllegalXPFClassID = 0,
     };
     static int32_t gCurrXPFClassID;
 
     typedef GrProgramElement INHERITED;
 };
 
 #endif