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

layout/xul/moz.build

@@ -68,5 +68,3 @@ LOCAL_INCLUDES += [
     "/dom/base",
     "/dom/xul",
 ]
-
-REQUIRES_UNIFIED_BUILD = True

layout/xul/nsBoxFrame.cpp

@@ -266,8 +266,6 @@ bool nsBoxFrame::GetInitialHAlignment(nsBoxFrame::Halignment& aHalign) {
         return false;
     }
   }
-
-  return false;
 }
 
 bool nsBoxFrame::GetInitialVAlignment(nsBoxFrame::Valignment& aValign) {
@@ -307,8 +305,6 @@ bool nsBoxFrame::GetInitialVAlignment(nsBoxFrame::Valignment& aValign) {
         return false;
     }
   }
-
-  return false;
 }
 
 void nsBoxFrame::GetInitialOrientation(bool& aIsHorizontal) {

layout/xul/nsDeckFrame.cpp

@@ -14,6 +14,7 @@
 #include "nsDeckFrame.h"
 #include "mozilla/ComputedStyle.h"
 #include "mozilla/PresShell.h"
+#include "nsLayoutUtils.h"
 #include "nsPresContext.h"
 #include "nsIContent.h"
 #include "nsCOMPtr.h"

layout/xul/nsMenuBarListener.cpp

@@ -19,6 +19,7 @@
 #include "mozilla/Preferences.h"
 #include "mozilla/StaticPrefs_ui.h"
 #include "mozilla/TextEvents.h"
+#include "mozilla/dom/Document.h"
 #include "mozilla/dom/Event.h"
 #include "mozilla/dom/EventBinding.h"
 #include "mozilla/dom/KeyboardEvent.h"

widget/nsIRollupListener.h

@@ -9,6 +9,7 @@
 
 #include "nsTArray.h"
 #include "nsPoint.h"
+#include "Units.h"
 
 class nsIContent;
 class nsIWidget;