Auto merge of #19461 - jonleighton:issue-19171-4, r=KiChjang

Handle cases where selection API doesn't apply

The selection API only applies to certain <input> types:

https://html.spec.whatwg.org/multipage/#do-not-apply

This commit ensures that we handle that correctly.

Some notes:

1. TextControl::set_dom_selection_direction now calls
   set_selection_range(), which means that setting selectionDirection will
   now fire a selection event, as it should per the spec.

2. There is a test for the firing of the select event in
   tests/wpt/web-platform-tests/html/semantics/forms/textfieldselection/select-event.html,
   however the test did not run due to this syntax error:

   (pid:26017) "ERROR:script::dom::bindings::error: Error at http://web-platform.test:8000/html/semantics/forms/textfieldselection/select-event.html:50:11 missing = in const declaration"

   This happens due to the us of the "for (const foo of ...)" construct.
   Per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of
   this should actually work, so it's somewhat unsatisfying to have to
   change the test.

3. I removed tests/wpt/web-platform-tests/html/semantics/forms/textfieldselection/selection-not-application-textarea.html
   because it doesn't seem to add any extra value - the selection API
   always applies to textarea elements, and the API is tested elsewhere.

4. If an `<input>`'s type is unset, it defaults to a text, and the
   selection API applies. Also, if an `<input>`'s type is set to an
   invalid value, it defaults to a text too. This second case doesn't
   currently work, and I'll need to do more restructuring of the code in
   a future commit. See discussion with nox in IRC:
   https://mozilla.logbot.info/servo/20171201#c13946454-c13946594

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