Change parser behavior of <select><select>

This patch makes the HTML parser convert nested <select> tags into
</select> instead of </select><select> for backwards compatibility with
the old parser. This behavior will apply to any nested selects, such as
<select><div><select>.

This was recommended here:
whatwg/html#10557 (comment)

This will be tested here:
html5lib/html5lib-tests#178 (comment)

Change-Id: I7a39901582b96edc10deca8794b9ce117aa08229

html/semantics/forms/the-select-element/customizable-select/select-parsing.tentative.html

@@ -104,15 +104,15 @@
   const outerSelect = document.getElementById('nested1');
   const innerSelect = document.getElementById('expectafter1');
   const nextDiv = document.getElementById('expectafter1b');
-  assert_true(!!outerSelect && !!innerSelect);
-  assert_equals(outerSelect.nextSibling, innerSelect,'Nested select should be pushed out as the next sibling');
-  assert_equals(innerSelect.nextElementSibling, nextDiv,'Subsequent content is there too');
+  assert_true(!!outerSelect);
+  assert_equals(innerSelect, null,'Nested select should be removed');
+  assert_equals(outerSelect.nextElementSibling, nextDiv,'Subsequent content is there too');
 }, 'Nested selects should be retained 1');
 
 test(() => {
   const outerSelect = document.getElementById('nested2');
   const innerSelect = document.getElementById('expectafter2');
-  assert_true(!!outerSelect && !!innerSelect);
-  assert_equals(outerSelect.nextSibling, innerSelect,'Nested select should be pushed out as the next sibling');
+  assert_true(!!outerSelect);
+  assert_equals(innerSelect, null,'Nested select should be pushed out as the next sibling');
 }, 'Nested selects should be retained 2');
 </script>