Add cap and builder feature for multisample interpolation

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1722363002

Review URL: https://codereview.chromium.org/1722363002
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
index 1219d34..57b8ee9 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
@@ -31,7 +31,8 @@
      */
     enum GLSLFeature {
         kStandardDerivatives_GLSLFeature = kLastGLSLPrivateFeature + 1,
-        kPixelLocalStorage_GLSLFeature
+        kPixelLocalStorage_GLSLFeature,
+        kMultisampleInterpolation_GLSLFeature
     };
 
     /**