Fix flag-metadata.json entry

I don't have a @chromium.org address, so the @google.com is necessary.

Change-Id: Iad68b94a35e67582191ba81343707f64ede5f4be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574228
Commit-Queue: Jared Saul <jsaul@google.com>
Commit-Queue: Anne Lim <annelim@google.com>
Auto-Submit: Jared Saul <jsaul@google.com>
Reviewed-by: Anne Lim <annelim@google.com>
Cr-Commit-Position: refs/heads/master@{#652311}