fix(chips): set appropriate aria-orientation (#6464)

Based on the [accessibility guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/#Listbox), the `listbox` role has a default `aria-orientation` of `vertical`, however the chips in a chip list are horizontal.

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

@@ -102,7 +102,7 @@ <h4>Stacked Chips</h4>
         <code>(focus)</code> event to run custom code.
       </p>
 
-      <md-chip-list class="mat-chip-list-stacked" [tabIndex]="-1">
+      <md-chip-list class="mat-chip-list-stacked" aria-orientation="vertical" [tabIndex]="-1">
         <md-chip *ngFor="let aColor of availableColors"
                  (focus)="color = aColor.color" color="{{aColor.color}}" selected="true">
           {{aColor.name}}

src/lib/chips/chip-list.ts

@@ -45,6 +45,7 @@ import {coerceBooleanProperty} from '@angular/cdk/coercion';
   host: {
     '[attr.tabindex]': '_tabIndex',
     'role': 'listbox',
+    '[attr.aria-orientation]': 'ariaOrientation',
     'class': 'mat-chip-list',
 
     '(focus)': 'focus()',
@@ -88,6 +89,9 @@ export class MdChipList implements AfterContentInit, OnDestroy {
   /** The chip components contained within this chip list. */
   chips: QueryList<MdChip>;
 
+  /** Orientation of the chip list. */
+  @Input('aria-orientation') ariaOrientation: 'horizontal' | 'vertical' = 'horizontal';
+
   constructor(protected _renderer: Renderer2, protected _elementRef: ElementRef,
               @Optional() private _dir: Directionality) {
   }

src/lib/chips/chips.md

@@ -33,10 +33,21 @@ chips are neither selectable nor focusable. Currently, disabled chips receive no
 Users can move through the chips using the arrow keys and select/deselect them with the space. Chips
 also gain focus when clicked, ensuring keyboard navigation starts at the appropriate chip.
 
+### Orientation
+If you want the chips in the list to be stacked vertically, instead of horizontally, you can apply
+the `mat-chip-list-stacked` class, as well as the `aria-orientation="vertical"` attribute:
+
+```html
+<md-chip-list class="mat-chip-list-stacked" aria-orientation="vertical">
+  <md-chip>Papadum</md-chip>
+  <md-chip>Naan</md-chip>
+  <md-chip>Dal</md-chip>
+</md-chip-list>
+```
 
 ### Theming
 The selected color of an `<md-chip>` can be changed by using the `color` property. By default, chips
-use a neutral background color based on the current theme (light or dark). This can be changed to 
+use a neutral background color based on the current theme (light or dark). This can be changed to
 `'primary'`, `'accent'`, or `'warn'`.
 
 ### Accessibility