Use vertexless shaders when NVpr is available

src/gpu/gl/GrGLProgramDesc.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLProgramDesc_DEFINED
 #define GrGLProgramDesc_DEFINED
 
@@ skipping 215 matching lines @@
     };
 
     SkAutoSMalloc<kPreAllocSize> fKey;
     bool fInitialized;
 
     // GrGLProgram and GrGLShaderBuilder read the private fields to generate code. TODO: Move all
     // code generation to GrGLShaderBuilder (and maybe add getters rather than friending).
     friend class GrGLProgram;
     friend class GrGLShaderBuilder;
     friend class GrGLFullShaderBuilder;
+    friend class GrGLFragmentOnlyShaderBuilder;
 };
 
 #endif