Add --webkit option as a shorthand for -s third_party/WebKit.

git-try

Index: git-try
diff --git a/git-try b/git-try
index bd78ebf957c7dbf714e7deb3745311f5132e91f2..713372bf67bdb2bd2dc106e1e6b9cff6a48a9818 100755
--- a/git-try
+++ b/git-try
@@ -4,6 +4,7 @@
 # found in the LICENSE file.
 """Wrapper for trychange.py for git checkout."""
 
+import logging
 import sys
 
 try:
@@ -35,4 +36,6 @@ if __name__ == '__main__':
         '--issue', GetRietveldIssueNumber(),
         '--patchset', patchset,
     ])
+  # Hack around a limitation in logging.
+  logging.getLogger().handlers = []
   sys.exit(trychange.TryChange(args, [], False, 'git-try'))