Add some backend flexibility for shader declarations in ProgramBuilding.

src/gpu/gl/GrGLVaryingHandler.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 GrGLVaryingHandler_DEFINED
 #define GrGLVaryingHandler_DEFINED
 
 #include "glsl/GrGLSLVarying.h"
 #include "GrTypesPriv.h"
 #include "gl/GrGLProgramDataManager.h"
 
 class GrGLVaryingHandler : public GrGLSLVaryingHandler {
 public:
     GrGLVaryingHandler(GrGLSLProgramBuilder* program)
         : INHERITED(program),
         fPathProcVaryingInfos(kVaryingsPerBlock) {}
 
     // This function is used by the NVPR PathProcessor to add a varying directly into the fragment
     // shader since there is no vertex shader.
     VaryingHandle addPathProcessingVarying(const char* name, GrGLSLVertToFrag*,
                                            GrSLPrecision fsPrecision = kDefault_GrSLPrecision);
 
 private:
+    void onFinalize() override {}
+
     typedef GrGLProgramDataManager::VaryingInfo VaryingInfo;
     typedef GrGLProgramDataManager::VaryingInfoArray VaryingInfoArray;
 
     VaryingInfoArray fPathProcVaryingInfos;
 
     friend class GrGLProgramBuilder;
 
     typedef GrGLSLVaryingHandler INHERITED; 
 };
 
 #endif