Bug 1784265 - Remove XULElement.flex WebIDL API. r=mconley

It's basically an alias of setAttribute("flex", value), and it has no
remaining usage in the tree.

Since it's less useful now, let's remove the WebIDL API in favor of CSS.

Do this as a separate patch so that thunderbird / pine / etc can revert
this patch for diagnostics / to find UI with behavior changes.

Differential Revision: https://phabricator.services.mozilla.com/D154498

dom/webidl/XULElement.webidl

@@ -11,10 +11,6 @@ interface XULControllers;
 interface XULElement : Element {
   [HTMLConstructor] constructor();
 
-  // Layout properties
-  [SetterThrows]
-  attribute DOMString flex;
-
   // Properties for hiding elements.
   attribute boolean hidden;
   attribute boolean collapsed;

dom/xul/nsXULElement.cpp

@@ -846,28 +846,6 @@ nsresult nsXULElement::AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
                                        aSubjectPrincipal, aNotify);
 }
 
-void nsXULElement::GetFlex(DOMString& aValue) const {
-#ifdef DEBUG
-  printf_stderr("\n");
-  ListAttributes(stderr);
-  xpc_DumpJSStack(true, true, false);
-  printf_stderr("\n");
-  MOZ_ASSERT_UNREACHABLE("XULElement.flex getter");
-#endif
-  GetXULAttr(nsGkAtoms::flex, aValue);
-}
-
-void nsXULElement::SetFlex(const nsAString& aValue, ErrorResult& aRv) {
-#ifdef DEBUG
-  printf_stderr("\n");
-  ListAttributes(stderr);
-  xpc_DumpJSStack(true, true, false);
-  printf_stderr("\n");
-  MOZ_ASSERT_UNREACHABLE("XULElement.flex setter");
-#endif
-  SetXULAttr(nsGkAtoms::flex, aValue, aRv);
-}
-
 void nsXULElement::AddTooltipSupport() {
   nsXULTooltipListener* listener = nsXULTooltipListener::GetInstance();
   if (!listener) {

dom/xul/nsXULElement.h

@@ -398,9 +398,6 @@ class nsXULElement : public nsStyledElement {
   }
 
   // WebIDL API
-  void GetFlex(DOMString& aValue) const;
-  void SetFlex(const nsAString& aValue, mozilla::ErrorResult&);
-
   bool Hidden() const { return BoolAttrIsTrue(nsGkAtoms::hidden); }
   void SetHidden(bool aHidden) { SetXULBoolAttr(nsGkAtoms::hidden, aHidden); }
   bool Collapsed() const { return BoolAttrIsTrue(nsGkAtoms::collapsed); }