`PresShell::GetRootScrollFrameAsScrollable()` is equivalent to
`PresShell::GetRootScrollContainerFrame()`.
In ScrollContainerFrame.h, `DecideScrollableLayer()` has two versions, one has
four parameters, and the other has five parameters with the fifth parameter
`aDirtyRectHasBeenOverriden` having a default value `nullptr`. When we switch
the caller from `nsIScrollableFrame` to `ScrollContainerFrame`, we need to
remove the default value for the fifth parameter to avoid ambiguity.
Differential Revision: https://phabricator.services.mozilla.com/D211494
6306934820