Add cap and builder feature for multisample interpolation

src/gpu/glsl/GrGLSLFragmentShaderBuilder.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 GrGLSLFragmentShaderBuilder_DEFINED
 #define GrGLSLFragmentShaderBuilder_DEFINED
 
@@ skipping 13 matching lines @@
 public:
     GrGLSLFragmentBuilder(GrGLSLProgramBuilder* program) : INHERITED(program) {}
     virtual ~GrGLSLFragmentBuilder() {}
 
     /**
      * Use of these features may require a GLSL extension to be enabled. Shaders may not compile
      * if code is added that uses one of these features without calling enableFeature()
      */
     enum GLSLFeature {
         kStandardDerivatives_GLSLFeature = kLastGLSLPrivateFeature + 1,
-        kPixelLocalStorage_GLSLFeature
+        kPixelLocalStorage_GLSLFeature,
+        kMultisampleInterpolation_GLSLFeature
     };
 
     /**
      * If the feature is supported then true is returned and any necessary #extension declarations
      * are added to the shaders. If the feature is not supported then false will be returned.
      */
     virtual bool enableFeature(GLSLFeature) = 0;
 
     /**
      * This returns a variable name to access the 2D, perspective correct version of the coords in
@@ skipping 191 matching lines @@
     // the program creator
     GrProcessor::RequiredFeatures fUsedProcessorFeatures;
     bool fHasReadDstColor;
 #endif
 
     friend class GrGLSLProgramBuilder;
     friend class GrGLProgramBuilder;
 };
 
 #endif