Bug 1507536 - Remove spacer in toolbar-menubar, and just make the menubar items flex instead. r=dao

This also adds a skipintoolbarset to the titlebar-buttonbox-container hbox to avoid a case
where an exception can be thrown from CustomizableUI.isWidgetRemovable, which expects all
direct descendants of toolbars to either have an ID, be one of a special set of node types,
or have skipintoolbarset="true" set on it.

Differential Revision: https://phabricator.services.mozilla.com/D12592

--HG--
extra : moz-landing-system : lando

browser/base/content/browser.xul

@@ -730,12 +730,11 @@ xmlns="http://www.w3.org/1999/xhtml"
                autohide="true"
 #endif
                context="toolbar-context-menu">
-        <toolbaritem id="menubar-items" align="center">
+        <toolbaritem id="menubar-items" align="start" flex="1">
 # The entire main menubar is placed into browser-menubar.inc, so that it can be
 # shared with other top level windows in macWindow.inc.xul.
 #include browser-menubar.inc
         </toolbaritem>
-        <spacer flex="1" />
 #include titlebar-items.inc.xul
       </toolbar>
 

browser/base/content/titlebar-items.inc.xul

@@ -2,7 +2,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-<hbox class="titlebar-buttonbox-container">
+<hbox class="titlebar-buttonbox-container" skipintoolbarset="true">
   <hbox class="titlebar-buttonbox titlebar-color">
     <toolbarbutton class="titlebar-button titlebar-min" oncommand="window.minimize();"/>
     <toolbarbutton class="titlebar-button titlebar-max" oncommand="onTitlebarMaxClick();"/>