GClient should allow having 'url': None in .gclient

This is for situations where the solution is checked out by another SCM
mechanism (e.g. update-webkit or just svn co) but we want to use gclient to
fetch the dependencies.

Review URL: http://codereview.chromium.org/211057

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26876 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -670,7 +670,7 @@ class GClient(object):
         raise Error("solution %s specified more than once" % name)
       url = solution["url"]
       entries[name] = url
-      if run_scm:
+      if run_scm and url:
         self._options.revision = revision_overrides.get(name)
         scm = gclient_scm.CreateSCM(url, self._root_dir, name)
         scm.RunCommand(command, self._options, args, file_list)