Bug 1602915 - Remove nsIBrowserDOMWindow.isTabContentWindow. r=mossop,marionette-reviewers,whimboo

This only works for non-e10s and it also seems superseded by the
mPendingClose as per the comment.

Differential Revision: https://phabricator.services.mozilla.com/D92003

browser/base/content/browser.js

@@ -6359,10 +6359,6 @@ nsBrowserAccess.prototype = {
     );
   },
 
-  isTabContentWindow(aWindow) {
-    return gBrowser.browsers.some(browser => browser.contentWindow == aWindow);
-  },
-
   canClose() {
     return CanCloseWindow();
   },