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/window_control_placeholder_support.css

@@ -44,7 +44,6 @@ See the above repository for updates as well as full license text. */
 
 .titlebar-buttonbox{ color: var(--toolbar-color) }
 :root[sizemode="fullscreen"] .titlebar-buttonbox-container{ display: none }
-:root[sizemode="fullscreen"] #navigator-toolbox { position: relative; }
 
 :root[sizemode="fullscreen"] #TabsToolbar > .titlebar-buttonbox-container:last-child{
   position: absolute;