fix(chips): not visible in high contrast mode (#5338)

Fixes `md-chip` instances not being visible for users running high contrast mode.

src/lib/chips/chips.scss

@@ -1,3 +1,5 @@
+@import '../core/a11y/a11y';
+
 $mat-chip-vertical-padding: 8px;
 $mat-chip-horizontal-padding: 12px;
 
@@ -23,6 +25,10 @@ $mat-chips-chip-margin: $mat-chip-horizontal-padding / 4;
       margin: 0 $mat-chips-chip-margin 0 0;
     }
   }
+
+  @include cdk-high-contrast {
+    outline: solid 1px;
+  }
 }
 
 .mat-chip-list-stacked .mat-chip-list-wrapper {