Bug 1852963 - BrowserTestUtils.browserLoaded() should also accept gBrowser as input. r=dao

BrowserTestUtils.browserLoaded() wants a tabbrowser-tab as input, most of it will
also work passing gBrowser, and indeed some tests ended up doing that.
Unfortunately some parts of it are skipped in that case.
Since it's confusing anyway, let's make it more lenient on its input.

Differential Revision: https://phabricator.services.mozilla.com/D188212

Author: mak77

testing/mochitest/BrowserTestUtils/BrowserTestUtils.sys.mjs

@@ -440,6 +440,11 @@ export var BrowserTestUtils = {
       );
     }
 
+    // Consumers may pass gBrowser instead of a browser, so adjust for that.
+    if ("selectedBrowser" in browser) {
+      browser = browser.selectedBrowser;
+    }
+
     // If browser belongs to tabbrowser-tab, ensure it has been
     // inserted into the document.
     let tabbrowser = browser.ownerGlobal.gBrowser;