Correct iOS hermetic plist rule.

build/config/mac/base_rules.gni

 # 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.
 
 # This file contains rules that are shared between Mac and iOS.
 
 import("//build/toolchain/toolchain.gni")
 import("//build/config/mac/symbols.gni")
 
 if (is_mac) {
@@ skipping 27 matching lines @@
                              "deps",
                            ])
 
     script = "//build/config/mac/xcrun.py"
     sources = [
       invoker.source,
     ]
     outputs = [
       invoker.output,
     ]
-    args = [
+    args = []
+    if (!use_system_xcode) {
+      args += [
+        "--developer_dir",
+        hermetic_xcode_path,
+      ]
+    }
+    args += [
       "plutil",
       "-convert",
       invoker.format,
       "-o",
       rebase_path(invoker.output, root_build_dir),
       rebase_path(invoker.source, root_build_dir),
     ]
   }
 }
 
@@ skipping 173 matching lines @@
     ]
     if (!use_system_xcode) {
       args += [
         "--developer_dir",
         hermetic_xcode_path,
       ]
     }
     args += ibtool_flags
   }
 }