bindings: Mark TraceWrapperV8ReferenceHolder as final

The final annotation becomes necessary when TraceWrapperV8ReferenceHolder is
not trivially destructible any longer (which will be required temporary).

Change-Id: I4030ace34ee1c9ab0a620d36c8e7b800c2cea175
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841556
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703132}

third_party/blink/renderer/bindings/core/v8/trace_wrapper_v8_reference_test.cc

@@ -14,7 +14,7 @@ namespace {
 
 using TraceWrapperV8ReferenceTest = BindingTestSupportingGC;
 
-class TraceWrapperV8ReferenceHolder
+class TraceWrapperV8ReferenceHolder final
     : public GarbageCollected<TraceWrapperV8ReferenceHolder> {
  public:
   TraceWrapperV8ReferenceHolder() = default;