perf(module:modal): call focus() on the next rendering frame to prevent frame drop (#7293)

Calling `focus()` on the element causes browsers to do re-layouts. This can been seen here:
https://gist.github.com/paulirish/5d52fb081b3570c81e3a#setting-focus
Microtasks are executed asynchronously, but within the current rendering frame. Using `requestAnimationFrame`
except of `Promise.resolve` basically unloads the current frame and calls `focus()` on the next rendering
frame, this prevent frame drops on slower devices when the modal is opened.

Author: arturovt

components/modal/modal-container.directive.ts

@@ -23,6 +23,7 @@ import { fromEvent, Subject } from 'rxjs';
 import { takeUntil } from 'rxjs/operators';
 
 import { NzConfigService } from 'ng-zorro-antd/core/config';
+import { reqAnimFrame } from 'ng-zorro-antd/core/polyfill';
 import { NzSafeAny } from 'ng-zorro-antd/core/types';
 import { getElementOffset, isNotNil } from 'ng-zorro-antd/core/util';
 
@@ -157,7 +158,7 @@ export class BaseModalContainerComponent extends BasePortalOutlet implements OnD
     if (this.document) {
       this.elementFocusedBeforeModalWasOpened = this.document.activeElement as HTMLElement;
       if (this.host.nativeElement.focus) {
-        this.ngZone.runOutsideAngular(() => Promise.resolve().then(() => this.host.nativeElement.focus()));
+        this.ngZone.runOutsideAngular(() => reqAnimFrame(() => this.host.nativeElement.focus()));
       }
     }
   }

components/modal/modal.spec.ts

@@ -1121,7 +1121,7 @@ describe('NzModal', () => {
       });
 
       fixture.detectChanges();
-      flushMicrotasks();
+      tick(16);
 
       expect(document.activeElement!.tagName).toBe('NZ-MODAL-CONTAINER', 'Expected modal container to be focused.');
     }));