[api] Make dtor of v8::EmbedderHeapTracer public

This enables direct uses in e.g.
  unique_ptr<v8::EmbedderHeapTracer>

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I11f86c916dca5c31413866d9972178fccda7df9c
Reviewed-on: https://chromium-review.googlesource.com/1160538Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54883}

include/v8.h

@@ -7103,6 +7103,8 @@ class V8_EXPORT EmbedderHeapTracer {
     ForceCompletionAction force_completion;
   };
 
+  virtual ~EmbedderHeapTracer() = default;
+
   /**
    * Called by v8 to register internal fields of found wrappers.
    *
@@ -7194,8 +7196,6 @@ class V8_EXPORT EmbedderHeapTracer {
   };
 
  protected:
-  virtual ~EmbedderHeapTracer() = default;
-
   v8::Isolate* isolate_ = nullptr;
 
   friend class internal::LocalEmbedderHeapTracer;