Bug 1896516 Part 9 - Remove nsIScrollableFrame usages under dom/. r=layout-reviewers,emilio

Also, in EventStateManager, convert `ComputeScrollTarget()` and
`ComputeScrollTargetAndMayAdjustWheelEvent()` to return `ScrollContainerFrame*`
so that we can drop more unnecessarily `do_QueryFrame`.

Due to removing `nsIScrollableFrame` forward declaration in DOM headers, we have
to add `nsIScrollableFrame` forward declaration temporarily in
nsMenuPopupFrame.h to make this patch compile, but we'll remove it once we
remove `nsIScrollableFrame` under layout/.

Differential Revision: https://phabricator.services.mozilla.com/D211496

dom/html/nsGenericHTMLElement.cpp

@@ -32,7 +32,6 @@
 #include "mozilla/dom/UnbindContext.h"
 #include "nsPIDOMWindow.h"
 #include "nsIFrameInlines.h"
-#include "nsIScrollableFrame.h"
 #include "nsView.h"
 #include "nsViewManager.h"
 #include "nsIWidget.h"