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This updates the use of the |action_index| and |reply| values throughout
the notification event dispatching code to base::Optional<>, reflecting
that neither has to be present.

BUG=

Change-Id: I930cd6a103138e91ffb2f4f0bfdf074c87a0c44b
Reviewed-on: https://chromium-review.googlesource.com/655307Reviewed-by: John Mellor <johnme@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501290}