Add InputClosesSelect flag

This flag is intended to de-risk the launch of SelectParserRelaxation by
partially reverting the new parser behavior to the old parser behavior
specifically in the case of an <input> tag being parsed inside a
<select>. The old parser would convert <select><input> into
<select></select><input>, and based on my research, this is the case
that is most likely going to break sites in SelectParserRelaxation:
whatwg/html#10310

Bug: 373672164
Change-Id: I33b40d11c2001092aa076a219dd56c5ea86f13f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5936092
Reviewed-by: Mason Freed <[email protected]>
Commit-Queue: Joey Arhar <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1369676}

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

@@ -42,6 +42,10 @@
   </div>
 </select>
 
+<select id=s7>
+  <input>
+</select>
+
 <div id=afterlast>
   keep this div after the last test case
 </div>
@@ -100,6 +104,13 @@
 `);
 }, 'Divs and imgs should be allowed as direct children of select and within options without a datalist.');
 
+test(() => {
+  assert_equals(document.getElementById('s7').parentNode, document.body);
+  assert_equals(document.getElementById('s7').innerHTML, `
+  <input>
+`);
+}, 'Input tags should parse inside select instead of closing the select.');
+
 test(() => {
   assert_equals(document.getElementById('afterlast').parentNode, document.body);
 }, 'The last test should not leave any tags open after parsing.');