LSC Add 'Update Mechanism:' to README.chromium Bug: 417367156 Change-Id: I318629733d1f75262e030105ec0b684f1307d2fd Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6646426 Auto-Submit: Jordan Brown <[email protected]> Owners-Override: Jordan Brown <[email protected]> Commit-Queue: Jordan Brown <[email protected]> Commit-Queue: Jiewei Qian <[email protected]> Reviewed-by: Jiewei Qian <[email protected]> Reviewed-by: Rachael Newitt <[email protected]> Reviewed-by: Joey Scarr <[email protected]> Cr-Commit-Position: refs/heads/main@{#1475401} NOKEYCHECK=True GitOrigin-RevId: da752eb2a3660cf1bf8dac620f6380b89dd953a7