fix(chips): set appropriate aria-orientation

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.

Author: crisbeto

src/lib/chips/chip-list.ts

@@ -45,6 +45,7 @@ import {coerceBooleanProperty} from '@angular/cdk/coercion';
   host: {
     '[attr.tabindex]': '_tabIndex',
     'role': 'listbox',
+    'aria-orientation': 'horizontal',
     'class': 'mat-chip-list',
 
     '(focus)': 'focus()',