Pass path to gn binary to cronet_licenses.py when building for iOS.

components/cronet/ios/BUILD.gn

 # Copyright 2015 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/buildflag_header.gni")
 import("//build/config/ios/rules.gni")
 import("//build/config/mac/symbols.gni")
 import("//build/mac/tweak_info_plist.gni")
 import("//build/util/version.gni")
 import("//chrome/version.gni")
@@ skipping 128 matching lines @@
   ]
 }
 
 if (additional_toolchains == [] || current_toolchain == default_toolchain) {
   _package_dir = "$root_out_dir/cronet"
 
   action("generate_license") {
     _license_path = "$_package_dir/LICENSE"
 
     script = "//components/cronet/tools/cronet_licenses.py"
+    inputs = [
+      "//build/util/LASTCHANGE",
+      "//buildtools/$host_os/gn",
+    ]
     outputs = [
       _license_path,
     ]
     args = [
       "license",
       rebase_path(_license_path, root_build_dir),
       "--gn",
+      "--gn-path",
+      rebase_path("//buildtools/$host_os/gn", root_build_dir),
     ]
   }
 
   copy("cronet_package_copy") {
     sources = [
       "$root_out_dir/Cronet.framework",
       "//AUTHORS",
       "//chrome/VERSION",
     ]
     outputs = [
       "$_package_dir/{{source_file_part}}",
     ]
 
     deps = [
       ":cronet_framework",
     ]
   }
 
   group("cronet_package") {
     deps = [
       ":cronet_package_copy",
       ":generate_license",
     ]
   }
 }