Revert "perf(cdk/scrolling): do not run change detection if there are…

… no `viewChange` listeners (angular#23987)"

This reverts commit ad33d7a.

src/cdk/scrolling/virtual-for-of.ts

@@ -199,9 +199,7 @@ export class CdkVirtualForOf<T>
     });
     this._viewport.renderedRangeStream.pipe(takeUntil(this._destroyed)).subscribe(range => {
       this._renderedRange = range;
-      if (this.viewChange.observers.length) {
-        ngZone.run(() => this.viewChange.next(this._renderedRange));
-      }
+      ngZone.run(() => this.viewChange.next(this._renderedRange));
       this._onRenderedDataChange();
     });
     this._viewport.attach(this);

src/cdk/scrolling/virtual-scroll-viewport.spec.ts

@@ -15,7 +15,6 @@ import {
   ViewEncapsulation,
   Directive,
   ViewContainerRef,
-  ApplicationRef,
 } from '@angular/core';
 import {
   waitForAsync,
@@ -787,43 +786,6 @@ describe('CdkVirtualScrollViewport', () => {
         }
       }).not.toThrow();
     }));
-
-    describe('viewChange change detection behavior', () => {
-      let appRef: ApplicationRef;
-
-      beforeEach(inject([ApplicationRef], (ar: ApplicationRef) => {
-        appRef = ar;
-      }));
-
-      it('should not run change detection if there are no viewChange listeners', fakeAsync(() => {
-        finishInit(fixture);
-        testComponent.items = Array(10).fill(0);
-        fixture.detectChanges();
-        flush();
-
-        spyOn(appRef, 'tick');
-
-        viewport.scrollToIndex(5);
-        triggerScroll(viewport);
-
-        expect(appRef.tick).not.toHaveBeenCalled();
-      }));
-
-      it('should run change detection if there are any viewChange listeners', fakeAsync(() => {
-        testComponent.virtualForOf.viewChange.subscribe();
-        finishInit(fixture);
-        testComponent.items = Array(10).fill(0);
-        fixture.detectChanges();
-        flush();
-
-        spyOn(appRef, 'tick');
-
-        viewport.scrollToIndex(5);
-        triggerScroll(viewport);
-
-        expect(appRef.tick).toHaveBeenCalledTimes(1);
-      }));
-    });
   });
 
   describe('with RTL direction', () => {