refactor(cdk-experimental/scrolling): fix strict property initialization errors

Updates the code to be compatible with strict property initialization.

Author: crisbeto

src/cdk-experimental/scrolling/auto-size-virtual-scroll.ts

@@ -91,13 +91,13 @@ export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
   private _averager: ItemSizeAverager;
 
   /** The last measured scroll offset of the viewport. */
-  private _lastScrollOffset: number;
+  private _lastScrollOffset!: number;
 
   /** The last measured size of the rendered content in the viewport. */
-  private _lastRenderedContentSize: number;
+  private _lastRenderedContentSize!: number;
 
   /** The last measured size of the rendered content in the viewport. */
-  private _lastRenderedContentOffset: number;
+  private _lastRenderedContentOffset!: number;
 
   /**
    * The number of consecutive cycles where removing extra items has failed. Failure here means that

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

@@ -92,7 +92,7 @@ function finishInit(fixture: ComponentFixture<any>) {
   imports: [ScrollingModule, ExperimentalScrollingModule],
 })
 class AutoSizeVirtualScroll {
-  @ViewChild(CdkVirtualScrollViewport, {static: true}) viewport: CdkVirtualScrollViewport;
+  @ViewChild(CdkVirtualScrollViewport, {static: true}) viewport!: CdkVirtualScrollViewport;
 
   orientation: 'vertical' | 'horizontal' = 'vertical';
   viewportSize = 200;