fix(table): Allow any iterable to be used as for columns in headerrow and row defs (#10822)

Author: josephperrott

src/cdk/table/row.ts

@@ -32,7 +32,7 @@ export const CDK_ROW_TEMPLATE = `<ng-container cdkCellOutlet></ng-container>`;
  */
 export abstract class BaseRowDef {
   /** The columns to be displayed on this row. */
-  columns: string[];
+  columns: Iterable<string>;
 
   /** Differ used to check if any changes were made to the columns. */
   protected _columnsDiffer: IterableDiffer<any>;

src/cdk/table/table.ts

@@ -535,7 +535,7 @@ export class CdkTable<T> implements CollectionViewer, OnInit, AfterContentChecke
    */
   private _getHeaderCellTemplatesForRow(headerDef: CdkHeaderRowDef): CdkHeaderCellDef[] {
     if (!headerDef || !headerDef.columns) { return []; }
-    return headerDef.columns.map(columnId => {
+    return Array.from(headerDef.columns, columnId => {
       const column = this._columnDefsByName.get(columnId);
 
       if (!column) {
@@ -546,13 +546,14 @@ export class CdkTable<T> implements CollectionViewer, OnInit, AfterContentChecke
     });
   }
 
+
   /**
    * Returns the cell template definitions to insert in the provided row
    * as defined by its list of columns to display.
    */
   private _getCellTemplatesForRow(rowDef: CdkRowDef<T>): CdkCellDef[] {
     if (!rowDef.columns) { return []; }
-    return rowDef.columns.map(columnId => {
+    return Array.from(rowDef.columns, columnId => {
       const column = this._columnDefsByName.get(columnId);
 
       if (!column) {