Add a BUILD target and a script to build a public static library for ios/web_view.

ios/web_view/internal/hide_symbols.py

+#!/usr/bin/env python
+
+# Copyright 2017 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.
+
+# Create a static library which exposes only symbols which are explicitly marked
+# as visible e.g., by __attribute__((visibility("default"))).
+#
+# See BUILD.gn in this directory for usage example.
+#
+# This way, we can reduce risk of symbol conflict when linking it into apps
+# by exposing internal symbols, especially in third-party libraries.
+
+import optparse
+import subprocess
+
+
+def main():
+  parser = optparse.OptionParser()
+  parser.add_option(
+      '--input_lib',
+      help='The path to an input .a file which contains symbols which must be '
+           'always linked.')
+  parser.add_option(
+      '--deps_lib',
+      help='The path to a complete static library (.a file) which contains all '
+           'dependencies of --input_lib. .o files in this library are also '
+           'added to the output library, but only if they are referred from '
+           '--input_lib.')
+  parser.add_option(
+      '--output_obj',
+      help='Outputs the generated .o file here. This is an intermediate file.')
+  parser.add_option(
+      '--output_lib',
+      help='Outputs the generated .a file here.')
+  (options, args) = parser.parse_args()
+  assert not args
+
+  # ld -r concatenates multiple .o files and .a files into a single .o file,
+  # while "hiding" symbols not marked as visible.
+  command = [
+    'xcrun', 'ld', '-r',
+    # By default, ld only pulls .o files out of a static library if needed to
+    # resolve some symbol reference. We apply -force_load option to input_lib
+    # (but not to deps_lib) to force pulling all .o files.
+    '-force_load', options.input_lib,
+    options.deps_lib,
+    '-o', options.output_obj
+  ]
+  subprocess.check_call(command)
+
+  # Creates a .a file which contains a single .o file.
+  command = [
+    'xcrun', 'ar', '-r',
+    options.output_lib,
+    options.output_obj,
+  ]
+  subprocess.check_call(command)
+
+
+if __name__ == "__main__":
+  main()