Add _lock convenience method

This will get reused later

BUG=None
TEST=None

Change-Id: I00af69980c86e8c78b9eab18f263e3e70e3f30ff
Reviewed-on: https://chromium-review.googlesource.com/450909
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/cros_venv/venvlib.py b/cros_venv/venvlib.py
index 00c2cdd..258964c 100644
--- a/cros_venv/venvlib.py
+++ b/cros_venv/venvlib.py
@@ -44,6 +44,10 @@
     def _installed_reqs_file(self):
         return os.path.join(self._venv_dir, '.installed.txt')
 
+    def _lock(self):
+        """Return lock context for the virtualenv."""
+        return flock.FileLock(self._lock_file)
+
     @property
     def _site_packages(self):
         return os.path.join(
@@ -53,7 +57,7 @@
     def ensure(self):
         """Create or update virtualenv."""
         _makedirs_exist_ok(self._venv_dir)
-        with flock.FileLock(self._lock_file):
+        with self._lock():
             if not self._venv_initialized():
                 self._init_venv()
             if not self._reqs_up_to_date():