Make optimization of blink::SelectType easier

* Add |final| to SelectType subclasses so that compilers can omit vtbl
  lookup code

* Add |const| to SelectType::select_ to avoid getting its value from
  memory multiple times in a single SelectType method

This CL has no behavior changes.

Bug: 1052232
Change-Id: I6c941defd9c8a4f428abd2a62e673f026daddb2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057223
Auto-Submit: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741465}

third_party/blink/renderer/core/html/forms/select_type.cc

@@ -34,7 +34,7 @@
 
 namespace blink {
 
-class MenuListSelectType : public SelectType {
+class MenuListSelectType final : public SelectType {
  public:
   explicit MenuListSelectType(HTMLSelectElement& select) : SelectType(select) {}
 
@@ -77,7 +77,7 @@ void MenuListSelectType::DidSelectOption(
 
 // ============================================================================
 
-class ListBoxSelectType : public SelectType {
+class ListBoxSelectType final : public SelectType {
  public:
   explicit ListBoxSelectType(HTMLSelectElement& select) : SelectType(select) {}
 };

third_party/blink/renderer/core/html/forms/select_type.h

@@ -28,7 +28,7 @@ class SelectType : public GarbageCollected<SelectType> {
  protected:
   explicit SelectType(HTMLSelectElement& select);
 
-  Member<HTMLSelectElement> select_;
+  const Member<HTMLSelectElement> select_;
 
  private:
   SelectType(const SelectType&) = delete;