handle missing ResizeObserver!!

Author: kseamon

src/cdk/table/sticky-styler.ts

@@ -34,7 +34,7 @@ export const STICKY_DIRECTIONS: StickyDirection[] = ['top', 'bottom', 'left', 'r
  */
 export class StickyStyler {
   private _elemSizeCache = new WeakMap<HTMLElement, {width: number; height: number}>();
-  private _resizeObserver = window.ResizeObserver
+  private _resizeObserver = window?.ResizeObserver
     ? new window.ResizeObserver(entries => this._updateCachedSizes(entries))
     : null;
   private _updatedStickyColumnsParamsToReplay: UpdateStickyColumnsParams[] = [];