refactor(table): change _data property from private to protected (#11669)

This makes it easier to extend the CdkTable

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "material2-srcs",
-  "version": "6.2.0",
+  "version": "6.2.1",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {

src/cdk/table/table.ts

@@ -148,12 +148,12 @@ export interface RenderRow<T> {
   changeDetection: ChangeDetectionStrategy.OnPush,
 })
 export class CdkTable<T> implements AfterContentChecked, CollectionViewer, OnDestroy, OnInit {
+  /** Latest data provided by the data source. */
+  protected _data: T[];
+
   /** Subject that emits when the component has been destroyed. */
   private _onDestroy = new Subject<void>();
 
-  /** Latest data provided by the data source. */
-  private _data: T[];
-
   /** List of the rendered rows as identified by their `RenderRow` object. */
   private _renderRows: RenderRow<T>[];