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We ended up not using it, and there are no plans to use it in
the future. It adds some unnecessary complexity to
SyncMessageFilter which I don't want to have to maintain as
I add some extra support for associated interfaces.

BUG=612500
R=jam@chromium.org

Review-Url: https://codereview.chromium.org/2343033002
Cr-Commit-Position: refs/heads/master@{#419205}