Mark ServiceWorkerClientAttributes flag as stable.
Mark ServiceWorkerClientAttributes flag as stable.

Per blink-dev thread:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/sb4WHR4Wdbk/hz8iE6-7-YAJ

BUG=446843

Review URL: https://codereview.chromium.org/916703003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190073 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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