Add gl_SampleMask functionality to fragment builders

src/gpu/glsl/GrGLSLShaderBuilder.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 GrGLSLShaderBuilder_DEFINED
 #define GrGLSLShaderBuilder_DEFINED
 
@@ skipping 134 matching lines @@
     /**
      * Features that should only be enabled internally by the builders.
      */
     enum GLSLPrivateFeature {
         kFragCoordConventions_GLSLPrivateFeature,
         kBlendEquationAdvanced_GLSLPrivateFeature,
         kBlendFuncExtended_GLSLPrivateFeature,
         kExternalTexture_GLSLPrivateFeature,
         kFramebufferFetch_GLSLPrivateFeature,
         kNoPerspectiveInterpolation_GLSLPrivateFeature,
-        kLastGLSLPrivateFeature = kNoPerspectiveInterpolation_GLSLPrivateFeature
+        kSampleVariables_GLSLPrivateFeature,
+        kSampleMaskOverrideCoverage_GLSLPrivateFeature,
+        kLastGLSLPrivateFeature = kSampleMaskOverrideCoverage_GLSLPrivateFeature
     };
 
     /*
      * A general function which enables an extension in a shader if the feature bit is not present
+     *
+     * @return true if the feature bit was not yet present, false otherwise.
      */
-    void addFeature(uint32_t featureBit, const char* extensionName);
+    bool addFeature(uint32_t featureBit, const char* extensionName);
 
     enum InterfaceQualifier {
         kOut_InterfaceQualifier,
         kLastInterfaceQualifier = kOut_InterfaceQualifier
     };
 
     /*
      * A low level function to build default layout qualifiers.
      *
      *   e.g. layout(param1, param2, ...) out;
@@ skipping 54 matching lines @@
     int fCodeIndex;
     bool fFinalized;
 
     friend class GrGLSLProgramBuilder;
     friend class GrGLProgramBuilder;
     friend class GrGLSLVaryingHandler; // to access noperspective interpolation feature.
     friend class GrGLPathProgramBuilder; // to access fInputs.
     friend class GrVkProgramBuilder;
 };
 #endif