feat(material/chips): update chip-list describedby to match input

Fixes ExpressionChangedAfterItHasBeenCheckedError that occurs when chip-lists are content-projected into a mat-form-field

Author: RobertAKARobin

src/material/chips/chip-list.ts

@@ -78,7 +78,6 @@ export class MatChipListChange {
   exportAs: 'matChipList',
   host: {
     '[attr.tabindex]': 'disabled ? null : _tabIndex',
-    '[attr.aria-describedby]': '_ariaDescribedby || null',
     '[attr.aria-required]': 'role ? required : null',
     '[attr.aria-disabled]': 'disabled.toString()',
     '[attr.aria-invalid]': 'errorState',
@@ -144,9 +143,6 @@ export class MatChipList
   /** Uid of the chip list */
   _uid: string = `mat-chip-list-${nextUniqueId++}`;
 
-  /** The aria-describedby attribute on the chip list for improved a11y. */
-  _ariaDescribedby: string;
-
   /** Tab index for the chip list. */
   _tabIndex = 0;
 
@@ -455,7 +451,11 @@ export class MatChipList
    * @docs-private
    */
   setDescribedByIds(ids: string[]) {
-    this._ariaDescribedby = ids.join(' ');
+    if (ids.length) {
+      this._elementRef.nativeElement.setAttribute('aria-describedby', ids.join(' '));
+    } else {
+      this._elementRef.nativeElement.removeAttribute('aria-describedby');
+    }
   }
 
   // Implemented as part of ControlValueAccessor.

tools/public_api_guard/material/chips.md

@@ -181,7 +181,6 @@ export interface MatChipInputEvent {
 export class MatChipList extends _MatChipListBase implements MatFormFieldControl<any>, ControlValueAccessor, AfterContentInit, DoCheck, OnInit, OnDestroy, CanUpdateErrorState {
     constructor(_elementRef: ElementRef<HTMLElement>, _changeDetectorRef: ChangeDetectorRef, _dir: Directionality, _parentForm: NgForm, _parentFormGroup: FormGroupDirective, _defaultErrorStateMatcher: ErrorStateMatcher, ngControl: NgControl);
     _allowFocusEscape(): void;
-    _ariaDescribedby: string;
     ariaOrientation: 'horizontal' | 'vertical';
     _blur(): void;
     readonly change: EventEmitter<MatChipListChange>;