Bug 1754105: Fix non-unified build errors in layout/xul, and mark it as safe to build in non-unified mode. r=emilio

Build errors being fixed here:

layout/xul/nsDeckFrame.cpp:164:43: error: unknown type name 'nsSetAttrRunnable'

layout/xul/nsMenuBarListener.cpp:45:7: error: cannot initialize a member subobject of type 'mozilla::dom::EventTarget *' with an rvalue of type 'nsINode::Document *' (aka 'mozilla::dom::Document *')
(...where "Document" is not defined; this patch includes the header that defines it, which fixes this.)

layout/xul/nsBoxFrame.cpp:270:10: error: 'return' will never be executed [-Werror,-Wunreachable-code-return]

dist/include/nsIRollupListener.h:36:38: error: no type named 'LayoutDeviceIntPoint' in namespace 'mozilla'
(For this error, this patch is changing a header outside of layout/xul, but
it's still required in order for layout/xul to build properly, since it's
#included by layout/xul/nsXULPopupManager.cpp (indirectly, via its .h file).

Differential Revision: https://phabricator.services.mozilla.com/D138092