Docserver: Fix invalid path usage in CloudStorageFileSystem.

chrome/common/extensions/docs/server2/path_util.py

Index: chrome/common/extensions/docs/server2/path_util.py
diff --git a/chrome/common/extensions/docs/server2/path_util.py b/chrome/common/extensions/docs/server2/path_util.py
index 06fa75e5294a49df16a0bcfc368f7fa1e50d78a5..fa9db48553dc82f86a333aa9171b48fb88dd8d66 100644
--- a/chrome/common/extensions/docs/server2/path_util.py
+++ b/chrome/common/extensions/docs/server2/path_util.py
@@ -29,6 +29,11 @@ def AssertIsValid(path):
   assert IsValid(path), 'Path "%s" is invalid' % path
 
 
+def Join(*paths):
+  assert all(IsValid(path) for path in paths), paths
+  return posixpath.join(*paths)
+
+
 def SplitParent(path):
   '''Returns the parent directory and base name of |path| in a tuple.
   Any trailing slash of |path| is preserved, such that the parent of