Add BUILD.gn for ios/third_party/fishhook.

Create GN build files for fishhook.

BUG=589100

Review URL: https://codereview.chromium.org/1732543002

Cr-Commit-Position: refs/heads/master@{#377372}

BUILD.gn

@@ -206,6 +206,7 @@ group("both_gn_and_gyp") {
       "//ios/public/provider/chrome/browser",
       "//ios/public/provider/web",
       "//ios/testing:ocmock_support_unittest",
+      "//ios/third_party/fishhook",
       "//ios/third_party/ochamcrest",
       "//ios/web:ios_web_unittests",
       "//ios/web/shell:ios_web_shell",

ios/third_party/fishhook/BUILD.gn

+# Copyright 2016 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.
+
+config("fishhook_config") {
+  include_dirs = [ "src" ]
+  visibility = [ ":fishhook" ]
+}
+
+source_set("fishhook") {
+  testonly = true
+  sources = [
+    "src/fishhook.c",
+    "src/fishhook.h",
+  ]
+
+  public_configs = [ ":fishhook_config" ]
+  configs += [ ":fishhook_config" ]
+}

ios/third_party/fishhook/fishhook.gyp

@@ -5,6 +5,7 @@
 {
   'targets': [
     {
+      # GN version: //ios/third_party/fishhook
       'target_name': 'fishhook',
       'type': 'static_library',
       'sources': [