define security critical attribute in third_party/schema

It is not packaged in Chromium/Chrome so I assume that implies
it's not security critical. Moreover, the library[1] doesn't seem
to do anything mentioned in [2]

[1]:https://github.com/keleshev/schema
[2]: https://chromium.googlesource.com/chromium/src/+/HEAD/docs/adding_to_third_party.md#add-a-readme_chromium

Bug: 365321156
Change-Id: I824b15feedb19ee3d1a7fa6d78bc2c9a432214b5
Fixed: 365321156
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5844683
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Joanna Wang <jojwang@chromium.org>

third_party/schema/README.chromium

@@ -4,6 +4,7 @@ Version: 0.6.6
 License: MIT
 License File: LICENSE-MIT
 Shipped: no
+Security Critical: no
 
 Description:
 schema is a library for validating Python data structures, such as those