`eContextMenu` event may be fired from `widget`. Therefore, different from
`ePointerClick` and `ePointerAuxClick`, they may cross the process boundary,
may be handled by APZ and may be dispatched into the DOM after a delay.
Therefore, this patch is complicated than the previous patch. This adds
* New IPC message handlers for sending/receiving a `WidgetPointerEvent`
* New `DelayedPointerEvent` class and templated `MouseInput::ToWidgetEvent`
* `PresShell::EventHandler` handles `eContextMenu` as same as `WidgetMouseEvent`
Differential Revision: https://phabricator.services.mozilla.com/D213003
d835b38b9f