Add gpu implementation of OverdrawXfermode

src/gpu/glsl/GrGLSLXferProcessor.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 GrGLSLXferProcessor_DEFINED
 #define GrGLSLXferProcessor_DEFINED
 
@@ skipping 57 matching lines @@
      */
     void setData(const GrGLSLProgramDataManager& pdm, const GrXferProcessor& xp);
 
 protected:
     static void DefaultCoverageModulation(GrGLSLXPFragmentBuilder* fragBuilder,
                                           const char* srcCoverage,
                                           const char* dstColor,
                                           const char* outColor,
                                           const char* outColorSecondary,
                                           const GrXferProcessor& proc);
-    
+
 private:
     /**
      * Called by emitCode() when the XP will not be performing a dst read. This method is
      * responsible for both blending and coverage. A subclass only needs to implement this method if
      * it can construct a GrXferProcessor that will not read the dst color.
      */
     virtual void emitOutputsForBlendState(const EmitArgs&) {
         SkFAIL("emitOutputsForBlendState not implemented.");
     }
 
@@ skipping 12 matching lines @@
                                          const GrXferProcessor&) {
         SkFAIL("emitBlendCodeForDstRead not implemented.");
     }
 
     virtual void onSetData(const GrGLSLProgramDataManager&, const GrXferProcessor&) = 0;
 
     GrGLSLProgramDataManager::UniformHandle fDstTopLeftUni;
     GrGLSLProgramDataManager::UniformHandle fDstScaleUni;
 };
 #endif