Add PR step to sync Chromium branches.

[briantting]

Rather than attempt to force push the cherry picks of m138 refresh commits, create pull requests instead.

Bug: 481788398

[github-actions]

🤖 Gemini Suggested Commit Message

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ci: Add PR creation step for Chromium sync

Rather than force-pushing cherry-picked Chromium commits directly
to the main branch, this change modifies the synchronization
workflow to create a pull request. This allows for proper review
and avoids potential issues with direct force-pushes.

Bug: 481788398

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💡 Pro Tips for a Better Commit Message:

1. Influence the Result: Want to change the output? You can write custom prompts or instructions directly in the Pull Request description. The model uses that text to generate the message.
2. Re-run the Generator: Post a comment with: /generate-commit-message

[briantting]

https://github.com/youtube/cobalt_sandbox/actions/runs/21925631384/job/63317291032
youtube/cobalt_sandbox#830

tested on sandbox