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Interface control messages were added before union support was ready so
the structs used for messages manually imitate unions. Now that union
support is ready, this workaround is no longer necessary and the structs
used for interface control messages can contain unions of the possible
message types.

Review-Url: https://codereview.chromium.org/2277853003
Cr-Commit-Position: refs/heads/master@{#414775}