[GTK] Extraneous empty bar shown in detached inspector windows

https://bugs.webkit.org/show_bug.cgi?id=255585

Reviewed by Carlos Garcia Campos.

The WI.undockedTitleAreaHeight() function only returns non-zero values
for Mac, but in Variables.css the --undocked-title-area-height variable
is set always to a non-zero value, the exact value depending on the
MacOS version. Add an :is(.mac-platform) match to leave the value
unchanged from zero in any non-Mac platform to keep the value in sync
with what WI.undockedTitleAreaHeight() returns.

* Source/WebInspectorUI/UserInterface/Views/Variables.css: Only change
  the --undocked-title-area-height variable for Mac.

Canonical link: https://commits.webkit.org/278418@main

Source/WebInspectorUI/UserInterface/Views/Variables.css

@@ -424,7 +424,7 @@ body {
         }
     }
 
-    &:not(.mac-platform.monterey, .mac-platform.big-sur):not(.docked) {
+    &:is(.mac-platform):not(.mac-platform.monterey, .mac-platform.big-sur):not(.docked) {
         /* keep in sync with `WI.undockedTitleAreaHeight` */
         --undocked-title-area-height: calc(22px / var(--zoom-factor));
     }