Add 'Update Mechanism: Manual' to README.chromium files Bug: 445311061 Change-Id: I2b756ad5795b793be54855c8e1883a2a2d8bbefd Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6953660 Reviewed-by: Jiewei Qian <[email protected]> Auto-Submit: Jordan Brown <[email protected]> Reviewed-by: Rachael Newitt <[email protected]> Commit-Queue: Jordan Brown <[email protected]> Owners-Override: Jordan Brown <[email protected]> Cr-Commit-Position: refs/heads/main@{#1516347} NOKEYCHECK=True GitOrigin-RevId: 305b2fe1fa7a2c66e0ffee71ff6cdab160c570a8