fix(material/chips): creates default aria-placeholder for chips input

Updates Angular Components Chips input so that if no placeholder value
is provided and if no ariaPlaceholder value is provided, that a default
value will be used. This improves accessibility by providing an
aria-placeholder value for VoiceControl to use as a name for the input.

Fixes b/380092814

Author: essjay05

src/dev-app/chips/chips-demo.html

@@ -209,7 +209,8 @@ <h4>Input is next sibling child of chip grid</h4>
         <input [matChipInputFor]="chipGrid2"
                [matChipInputSeparatorKeyCodes]="separatorKeysCodes"
                [matChipInputAddOnBlur]="addOnBlur"
-               (matChipInputTokenEnd)="add($event)" />
+               (matChipInputTokenEnd)="add($event)"
+               ariaPlaceholder="New contributor input..." />
       </mat-form-field>
 
       <p>

src/material/chips/chip-input.ts

@@ -59,6 +59,7 @@ export interface MatChipInputEvent {
     '[id]': 'id',
     '[attr.disabled]': 'disabled || null',
     '[attr.placeholder]': 'placeholder || null',
+    '[attr.aria-placeholder]': 'getAriaPlaceholder()',
     '[attr.aria-invalid]': '_chipGrid && _chipGrid.ngControl ? _chipGrid.ngControl.invalid : null',
     '[attr.aria-required]': '_chipGrid && _chipGrid.required || null',
     '[attr.required]': '_chipGrid && _chipGrid.required || null',
@@ -104,6 +105,9 @@ export class MatChipInput implements MatChipTextControl, OnChanges, OnDestroy {
   /** The input's placeholder text. */
   @Input() placeholder: string = '';
 
+  /** Screenreader placeholder for the input, only used if placeholder is not provided. */
+  @Input() ariaPlaceholder: string | null;
+
   /** Unique id for the input. */
   @Input() id: string = inject(_IdGenerator).getId('mat-mdc-chip-list-input-');
 
@@ -125,6 +129,10 @@ export class MatChipInput implements MatChipTextControl, OnChanges, OnDestroy {
   /** The native input element to which this directive is attached. */
   readonly inputElement!: HTMLInputElement;
 
+  /** Default Screen-reader placeholder for the input if no placeholder or
+   * ariaPlaceholder is provided. */
+  private readonly _defaultAriaPlaceholder = 'Enter input';
+
   constructor(...args: unknown[]);
 
   constructor() {
@@ -223,4 +231,11 @@ export class MatChipInput implements MatChipTextControl, OnChanges, OnDestroy {
   private _isSeparatorKey(event: KeyboardEvent) {
     return !hasModifierKey(event) && new Set(this.separatorKeyCodes).has(event.keyCode);
   }
+
+  /** Checks whether placeholder is used, if not checks for ariaPlaceholder, and resorts
+   * to default value if neither is provided.
+   */
+  getAriaPlaceholder(): string | null {
+    return this.placeholder ? null : this.ariaPlaceholder || this._defaultAriaPlaceholder;
+  }
 }