[ios] Pass missing variable substitution values.

https://codereview.chromium.org/2367923002/ tries to change
build/config/mac/plist_util.py to fails if some variables
have no value while performing substitution.

Fix invocations of plist_util.py when building Chrome on iOS
to allow the change to plist_util.py.

BUG=650345

Change-Id: Ibbd38373d851bb418e0328f5113342cdbc3604e9
Reviewed-on: https://chromium-review.googlesource.com/521106Reviewed-by: Louis Romero <lpromero@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476315}

ios/clean/chrome/app/BUILD.gn

@@ -53,12 +53,14 @@ ios_app_bundle("chrome_clean_skeleton") {
 
   extra_substitutions = [
     "CHROMIUM_BUNDLE_ID=$output_name",
+    "CHROMIUM_HANDOFF_ID=$chromium_handoff_id",
     "CHROMIUM_SHORT_NAME=$output_name",
     "CHROMIUM_URL_SCHEME_1=$url_unsecure_scheme",
     "CHROMIUM_URL_SCHEME_2=$url_secure_scheme",
     "CHROMIUM_URL_SCHEME_3=$url_x_callback_scheme",
     "CHROMIUM_URL_SCHEME_4=$url_channel_scheme",
     "IOS_BUNDLE_ID_PREFIX=$ios_app_bundle_id_prefix",
+    "SSOAUTH_URL_SCHEME=$url_ssoauth_scheme",
   ]
 
   if (ios_encryption_export_compliance_code != "") {