Fix --webkit trychange.py option.

trychange.py

Index: trychange.py
diff --git a/trychange.py b/trychange.py
index 2f3432641887b4d35e6443dc076a38a76cc610bf..96f72b8689c7418e4a0268c7b9a94a74b5f54de2 100755
--- a/trychange.py
+++ b/trychange.py
@@ -513,6 +513,8 @@ def TryChange(argv,
   # Mostly chromium-specific
   try:
     def WebKitRevision(options, opt, value, parser):
+      if not hasattr(options, 'sub_rep'):
+        options.sub_rep = []
       if parser.rargs and not parser.rargs[0].startswith('-'):
         options.sub_rep.append('third_party/WebKit@%s' % parser.rargs.pop(0))
       else: