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There are only ever a couple observers so base::flat_set should
have less overhead that std::unordered_set, especially given how
frequently it's copied.

Change-Id: Ib800e3461dbbb902041584ce8c4325eec4a91694
Reviewed-on: https://chromium-review.googlesource.com/c/1301965
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603161}