Correct iOS hermetic plist rule.

BUG=632229

Review-Url: https://codereview.chromium.org/2533613002
Cr-Original-Commit-Position: refs/heads/master@{#435135}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c49104d2d37cd6ad1c876ba8865b976f91a0ed98

config/mac/base_rules.gni

@@ -45,7 +45,14 @@ template("convert_plist") {
     outputs = [
       invoker.output,
     ]
-    args = [
+    args = []
+    if (!use_system_xcode) {
+      args += [
+        "--developer_dir",
+        hermetic_xcode_path,
+      ]
+    }
+    args += [
       "plutil",
       "-convert",
       invoker.format,