Fixes clang warning in GLSurfaceOzone.

Introduced in https://codereview.chromium.org/938873002/

R=piman@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/967343003

Cr-Commit-Position: refs/heads/master@{#318826}

ui/gl/gl_surface_ozone.cc

@@ -305,6 +305,8 @@ class GL_EXPORT GLSurfaceOzoneSurfacelessSurfaceImpl
     }
 
    private:
+    ~SurfaceImage() override {}
+
     scoped_refptr<ui::NativePixmap> pixmap_;
   };