Including extra_gn_args to log line.

In a recent commit we added --extra-gn-args flag but we forgot to log
the extra_gn_args variable and this can cause useless investigations.

BUG=None
NOTRY=True

Review-Url: https://codereview.webrtc.org/2698613004
Cr-Commit-Position: refs/heads/master@{#16639}
diff --git a/tools-webrtc/ios/build_ios_libs.py b/tools-webrtc/ios/build_ios_libs.py
index 3740483..25745e0 100755
--- a/tools-webrtc/ios/build_ios_libs.py
+++ b/tools-webrtc/ios/build_ios_libs.py
@@ -115,10 +115,10 @@
   else:
     raise ValueError('Build type "%s" is not supported.' % build_type)
 
-  logging.info('Building WebRTC with args: %s', ' '.join(gn_args))
+  args_string = ' '.join(gn_args + extra_gn_args)
+  logging.info('Building WebRTC with args: %s', args_string)
 
-  cmd = ['gn', 'gen', output_dir,
-         '--args=' + ' '.join(gn_args + extra_gn_args)]
+  cmd = ['gn', 'gen', output_dir, '--args=' + args_string]
   _RunCommand(cmd)
   logging.info('Building target: %s', gn_target_name)