[SelectMenu] Use option.value for selectmenu.value

This CL updates the behavior so that option.value attribute is used for
selectmenu.value. When the value attribute is not specified, the option
content will be used instead.

Since after [1], the option parts are sync updated, there is no need
to ensure that the selected option is valid before being used.

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/3285630

Bug: 1121840
Change-Id: I2b828d77fe7306eec22f99938e10666004532524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3315596
Reviewed-by: Dan Clark <[email protected]>
Reviewed-by: Mason Freed <[email protected]>
Commit-Queue: Ionel Popescu <[email protected]>
Cr-Commit-Position: refs/heads/main@{#948242}

html/semantics/forms/the-selectmenu-element/selectmenu-value.tentative.html

@@ -52,6 +52,12 @@
   <option>three</option>
 </selectmenu>
 
+<selectmenu id="selectMenu7">
+  <option id="selectMenu7-option1">one</option>
+  <option selected value="test">two</option>
+  <option>three</option>
+</selectmenu>
+
 <script>
 
 test(() => {
@@ -152,4 +158,20 @@
   assert_false(newOption.selected);
 }, "Test that HTMLOption.selected updates selectmenu.value");
 
+test(() => {
+  const selectMenu = document.getElementById("selectMenu7");
+  let selectMenuOption1 = document.getElementById("selectMenu7-option1");
+
+  assert_equals(selectMenu.value, "test");
+  assert_false(selectMenuOption1.selected);
+  selectMenuOption1.selected = true;
+  assert_equals(selectMenu.value, "one");
+  selectMenuOption1.value = "new test";
+  assert_equals(selectMenu.value, "new test");
+  selectMenuOption1.removeAttribute("value");
+  assert_equals(selectMenu.value, "one");
+  selectMenuOption1.value = "";
+  assert_equals(selectMenu.value, "");
+}, "Test that HTMLOption.value updates selectmenu.value");
+
 </script>