Bug 1461708 - part 3: EventStateManager::InitAndDispatchClickEvent() shouldn't overwrite nsEventStatus with nsEventStatus_eIgnore r=smaug

EventStateManager::InitAndDispatchClickEvent() sends given nsEventStatus to
nsIPresShell::HandleEventWithTarget().  Then, it sends the status to
EventStateManager::PreHandleEvent() before dispatching the event.  At this
time, EventStateManager::PreHandleEvent() resets the state to
nsEventStatus_eIgnore.  Therefore, for example, if eMouseClick event is
consumed but eMouseAuxClick is ignored, the event status result is
nsEventStatus_eIgnore.  So, neither DispatchClickEvents() nor
PostMouseUpEventHandler() cannot check whether at least one click event
is consumed.

This patch makes EventStateManager::InitAndDispatchClickEvent() sends
local variable of nsEventStatus to nsIPresShell::HandleEventWithTarget().
Then, merge the result with current status.

If we'd change nsEventStatus to enum class, we could make this change as
custom "operator|=" or something.

Differential Revision: https://phabricator.services.mozilla.com/D7850

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extra : moz-landing-system : lando