Force requirements modification to be locked.

... On my quest to make that stuff thread safe.

R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/8135008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@104033 0039d316-1c4b-4281-b951-d872f2087c98

gclient_utils.py

@@ -497,6 +497,10 @@ class WorkItem(object):
   def requirements(self):
     return tuple(self._requirements)
 
+  @lockedmethod
+  def add_requirement(self, new):
+    self._requirements.add(new)
+
 
 class ExecutionQueue(object):
   """Runs a set of WorkItem that have interdependencies and were WorkItem are