[OnionSoup] Remove struct content::PushEventPayload

Currently we have a [Native] mojom struct content.mojom.PushEventPayload
with a typemapping to the native struct content::PushEventPayload, we
use them to pass payload data when calling the Mojo call
content.mojom.ServiceWorker.DispatchPushEvent().

However, as content::PushEventPayload contains only 2 members: a string
containing real data and a bool indicating data's validity, and it has
no expectations to be expanded for now, so, we can just
use a base::Optional<string> to replace it, and correspondingly in mojom
we just needs a 'string? payload' for our
content.mojom.ServiceWorker.DispatchPushEvent().

This CL removes content::PushEventPayload by doing the above
replacement.

BUG=789854

Change-Id: I8df737165b3004db820e3b758c1bfa8bdb7b5690
Reviewed-on: https://chromium-review.googlesource.com/1149705
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#580445}
17 files changed