Bug 1259660 -  Moved mozilla::WidgetMosueEventBase::buttonType in MouseEvents.h to mozilla::MouseButton in EventForwards.h, and mozilla::WidgetMouseEventBase::buttonsFlag to mozilla::MouseButtonsFlag r=masayuki

Moved mozilla::WidgetMosueEventBase::buttonType in MouseEvents.h to mozilla::MouseButton in EventForwards.h, and mozilla::WidgetMouseEventBase::buttonsFlag to mozilla::MouseButtonsFlag so that any referer in header files do not need to include MouseEvents.h only for referring them. Instead, they just need to include EventForwards.h. Now when MouseEvents.h is changed, the rebuild speed becomes faster.

Differential Revision: https://phabricator.services.mozilla.com/D25325

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extra : moz-landing-system : lando