placeholder support: Don't set navigator-toolbox to position:relative

Instead, let the few styles that depend on it deal with it themselves by
setting position:relative to titlebar. Fixes #388

chrome/hide_tabs_with_one_tab_w_window_controls.css

@@ -26,6 +26,7 @@ Window controls will be all wrong without it
   transition: none !important;
   opacity: 1 !important;
 }
+:root[sizemode="fullscreen"] #titlebar{ position: relative }
 /* We'll use window controls from menubar instead */
 #TabsToolbar > .titlebar-buttonbox-container { display: none }
 /* Hide overflow button unless tabs overflow - is necessary for collpasing tabs with one tab */