More context:

I'm working on refactoring the Select-to-Speak event handler into an ash part and a chrome part -- it's currently all in Chrome. This will let it work under Mash / when ash and Chrome live in separate processes.

To do this I need to pass Mouse events handled in the ash part through a mojo interface to Chrome. The mojo event struct does not have any mouse event types (MOUSE_MOVED, etc).

Chatting over hangouts with sky@, he suggested supporting mouse events in mojo is a good solution.

+1 to serializing mouse events. In the beginning of mustash we tried to only use PointerEvent, but it's caused problems for us because we sometimes have to convert back and forth between PointerEvent and other event types, and sometimes we lose data in the conversion.

Yea, without a dedicated effort to switch to pointer-events, I think it'd make sense to just transfer mouse/touch events directly over mojo.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/71247087eae132067236517714f595d5132310a5

commit 71247087eae132067236517714f595d5132310a5
Author: Christopher Cameron <ccameron@chromium.org>
Date: Fri Sep 21 08:14:49 2018

Support Mouse event types in Mojo events.

This allows mouse type events to be passed through Mojo.

Bug:  881509 
Change-Id: I8bdcf980e7dce0fea5b71c285f86956cd334f7e5
Reviewed-on: https://chromium-review.googlesource.com/1211916
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593113}
[modify] https://crrev.com/71247087eae132067236517714f595d5132310a5/ui/events/mojo/event_constants.mojom
[modify] https://crrev.com/71247087eae132067236517714f595d5132310a5/ui/events/mojo/event_struct_traits.cc
[modify] https://crrev.com/71247087eae132067236517714f595d5132310a5/ui/events/mojo/struct_traits_unittest.cc