fix(scrolling): provide virtual scroll viewport as scrollable (#14168)

Since the `CdkVirtualScrollViewport` extends `CdkScrollable`, it should also provide itself as a `CdkScrollable` to DI.

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

@@ -50,6 +50,10 @@ function rangesEqual(r1: ListRange, r2: ListRange): boolean {
   },
   encapsulation: ViewEncapsulation.None,
   changeDetection: ChangeDetectionStrategy.OnPush,
+  providers: [{
+    provide: CdkScrollable,
+    useExisting: CdkVirtualScrollViewport,
+  }]
 })
 export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, OnDestroy {
   /** Emits when the viewport is detached from a CdkVirtualForOf. */