Bug 1930292 - Turn chromemargin into a boolean attribute. r=win-reviewers,desktop-theme-reviewers,pip-reviewers,firefox-desktop-core-reviewers ,dao,mconley,rkraesig

There are probably other simplifications that can be done after this on the
windows side of things, but I tried to keep this minimal.

Differential Revision: https://phabricator.services.mozilla.com/D229765

browser/base/content/browser-init.js

@@ -105,7 +105,8 @@ var gBrowserInit = {
     // Run menubar initialization first, to avoid TabsInTitlebar code picking
     // up mutations from it and causing a reflow.
     AutoHideMenubar.init();
-    // Update the chromemargin attribute so the window can be sized correctly.
+    // Update the customtitlebar attribute so the window can be sized
+    // correctly.
     window.TabBarVisibility.update();
     TabsInTitlebar.init();