Bug 1855280 - Remove the ability to switch to the old migration wizard. r=kpatenio,omc-reviewers,settings-reviewers,credential-management-reviewers,negin,aminomancer,mtigley

This removes the pref to re-enable the legacy wizard, and all of the things that used
to read from it.

It also disables the ability to use the legacy wizard from about:welcome with the
aboutWelcomeBehavior feature.

It also removes the closeMigrationWizard utility from BrowserTestUtils, since that
effectively became a thin-wrapper around BrowserTestUtils.removeTab.

Finally, this patch also removes some documentation about the legacy migration wizard,
and tests that specifically check the legacy dialog behaviour.

Differential Revision: https://phabricator.services.mozilla.com/D190716

browser/components/preferences/main.js

@@ -2164,22 +2164,14 @@ var gMainPane = {
   },
 
   onMigrationButtonCommand(command) {
-    // When browser.migrate.content-modal.enabled is enabled by default,
-    // the event handler can just call showMigrationWizardDialog directly,
-    // but for now, we delegate to MigrationUtils to open the native modal
-    // in case that's the dialog we're still using.
-    //
-    // Enabling the pref by default will be part of bug 1822156.
-    const browser = window.docShell.chromeEventHandler;
-    const browserWindow = browser.ownerGlobal;
+    // Even though we're going to be showing the migration wizard here in
+    // about:preferences, we'll delegate the call to
+    // `MigrationUtils.showMigrationWizard`, as this will allow us to
+    // properly measure entering the dialog from the PREFERENCES entrypoint.
+    const browserWindow = window.browsingContext.topChromeWindow;
 
-    // showMigrationWizard blocks on some platforms. We'll dispatch the request
-    // to open to a runnable on the main thread so that we don't have to block
-    // this function call.
-    Services.tm.dispatchToMainThread(() => {
-      MigrationUtils.showMigrationWizard(browserWindow, {
-        entrypoint: MigrationUtils.MIGRATION_ENTRYPOINTS.PREFERENCES,
-      });
+    MigrationUtils.showMigrationWizard(browserWindow, {
+      entrypoint: MigrationUtils.MIGRATION_ENTRYPOINTS.PREFERENCES,
     });
   },