fix(material/list): not working correctly when list item is used as a button

Based on our selector for `mat-list-item`, we allow it to be a `button`, however we don't do the necessary resets for it to be usable.

Author: crisbeto

src/material/list/list.scss

@@ -203,6 +203,14 @@ $item-inset-divider-offset: 72px;
   }
 }
 
+// We need some extra resets when the list items are buttons.
+button.mat-list-item, button.mat-list-option {
+  border: none;
+  background: none;
+  padding: 0;
+  width: 100%;
+}
+
 // This mixin adjusts the heights and padding based on whether the list is in dense mode.
 @mixin subheader-spacing($top-padding, $base-height) {
   height: $base-height;