Add gl_SampleMask functionality to fragment builders

src/gpu/glsl/GrGLSLShaderBuilder.h

Index: src/gpu/glsl/GrGLSLShaderBuilder.h
diff --git a/src/gpu/glsl/GrGLSLShaderBuilder.h b/src/gpu/glsl/GrGLSLShaderBuilder.h
index fa1b8e6c2f4f2ad7d5585a838fb815e15e9b6759..cc34e44f77221bf6219b4cfa345f0bbfd5f35fe0 100644
--- a/src/gpu/glsl/GrGLSLShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLShaderBuilder.h
@@ -131,14 +131,17 @@ protected:
         kBlendFuncExtended_GLSLPrivateFeature,
         kExternalTexture_GLSLPrivateFeature,
         kFramebufferFetch_GLSLPrivateFeature,
+        kSampleMaskOverrideCoverage_GLSLPrivateFeature,
         kNoPerspectiveInterpolation_GLSLPrivateFeature,
         kLastGLSLPrivateFeature = kNoPerspectiveInterpolation_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,