ssci: update diff metadata to include autoroll exemption

Bug: 422918273
Change-Id: I8005476d137d83eb78445ac0be6ac3a9d74aa393
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6963580
Auto-Submit: Jiewei Qian <qjw@chromium.org>
Commit-Queue: Jiewei Qian <qjw@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>

Author: wacky6

front_end/third_party/diff/README.chromium

@@ -6,6 +6,7 @@ Revision: 0ae308daa77aeddb089cd6b7b0a443fca026266e
 Update Mechanism: Manual
 License: Apache-2.0
 License File: LICENSE
+Update Mechanism: Static.HardFork (https://crbug.com/422918273)
 Security Critical: no
 Shipped: yes