Even if `<datalist>` is dynamically changed, the autocomplete controller still
uses the previous search result. If changed, we have to ignore the previous
result that may now be invalid.
Also, even if `<datalist>` is changed, we have to keep the selected index
(See Mozilla Bug 595069), so we cannot use `ResetInternalState` in this
situation because it resets the selected index.
This resolves#1509.
Moonchild
517aec3111