Add cap and builder feature for multisample interpolation

src/gpu/glsl/GrGLSLFragmentShaderBuilder.h

Index: src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
index 1219d34b463fb21ba79848f2cc4fab65f08c16a9..57b8ee91c60631d77f36536e2cacb9ec75e5f89e 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
@@ -31,7 +31,8 @@ public:
      */
     enum GLSLFeature {
         kStandardDerivatives_GLSLFeature = kLastGLSLPrivateFeature + 1,
-        kPixelLocalStorage_GLSLFeature
+        kPixelLocalStorage_GLSLFeature,
+        kMultisampleInterpolation_GLSLFeature
     };
 
     /**