Currently, we set coalesced events only when `eMouseMove` or `eTouchMove`
is received in a remote process. Therefore, synthesized events for tests and
`ePointerMove` event which is converted from non-`eMouseMove` event won't have
coalesced events.
This patch makes `PointerEvent::GetCoalescedEvents()` ensure at least one
coalesced event if it's a trust event, a `pointermove` event and its widget
event does not have coalesced events.
Ideally, we should fix this at synthesizer and `PointerEventHandler` to make
consistent data between user initiated events and synthesized events for tests
to check the behavior exactly. However, it requires much more complicated
patch and C++ default handlers shouldn't refer the coalesced events.
Therefore, this patch touches the path in `PointerEvent::GetCoalescedEvents()`.
Differential Revision: https://phabricator.services.mozilla.com/D220223
5034db3389