SELECT: Fix deselection behavior on leaving the "multiple" mode

If
 - the SELECT switched from the "multiple" mode,
 - it had two or more selected OPTIONs, and
 - last_on_change_option_ was the first selected OPTION,
we missed to deselect OPTIONs other than the first one due to the fast
path of DeselectItemsWithoutValidation().

This CL clears last_on_change_option_ before calling
DeselectItemsWithoutValidation().

Bug: 1245443
Change-Id: Ib9cc4a4770d99ed6782edc73fd8ff1fcb3d8738b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3205138
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#928412}

Author: tkent-google

html/semantics/forms/the-select-element/select-multiple.html

@@ -33,4 +33,16 @@
   assert_true(select.options[1].selected, "second option should be selected.");
 
 }, "multiple selected options exist, one set from script");
+
+// crbug.com/1245443
+test(() => {
+  let select = document.createElement("select");
+  select.length = 4;
+  let o1 = select.options.item(1);
+  select.multiple = true;
+  select.selectedIndex = 2;
+  o1.selected = true;
+  select.multiple = false;
+  assert_equals(select.selectedOptions.length, 1);
+}, "Removing multiple attribute reduces the number of selected OPTIONs to 1");
 </script>