Allows internal iOS code to extend the libxml visibility list.

BUG=699306

Change-Id: Ide0eba1f60a766ab0913642921dca725e28e258b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894016Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711818}

third_party/libxml/libxml_visibility.gni

+# Copyright 2019 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/compiler/compiler.gni")
+
+if (is_ios) {
+  declare_args() {
+    # iOS requires additional targets added to the libxml visibility list.
+    # Entries in this list must be strings relative to //ios_internal.
+    ios_libxml_visibility_additions = []
+  }
+}