deps: backport 4acdb5eec2c from upstream v8

Original commit message:

    Give v8::Eternal a direct reference to the handle.

    This makes it more similar to other handle types (like PersistentBase),
    by simply storing an i::Object** cast to T*. This means that it is not
    necessary to look up the handle in the eternal handles table to access
    the underlying value.

    Like the built-in roots (null, etc.), an eternal handle can never be
    destroyed, so we don't even need to allocate a separate local handle.
    Instead, the Local<T> can point directly at the eternal reference.
    This makes Eternal<T>::Get trivial.

    Review-Url: https://codereview.chromium.org/2751263003
    Cr-Commit-Position: refs/heads/master@{#43912}

Ref: v8/v8@4acdb5eec2c

PR-URL: #12875
Reviewed-By: James M Snell <jasnell@gmail.com>

deps/v8/include/v8.h

@@ -360,19 +360,18 @@ class MaybeLocal {
 // Eternal handles are set-once handles that live for the life of the isolate.
 template <class T> class Eternal {
  public:
-  V8_INLINE Eternal() : index_(kInitialValue) { }
-  template<class S>
-  V8_INLINE Eternal(Isolate* isolate, Local<S> handle) : index_(kInitialValue) {
+  V8_INLINE Eternal() : val_(nullptr) {}
+  template <class S>
+  V8_INLINE Eternal(Isolate* isolate, Local<S> handle) : val_(nullptr) {
     Set(isolate, handle);
   }
   // Can only be safely called if already set.
   V8_INLINE Local<T> Get(Isolate* isolate) const;
-  V8_INLINE bool IsEmpty() const { return index_ == kInitialValue; }
+  V8_INLINE bool IsEmpty() const { return val_ == nullptr; }
   template<class S> V8_INLINE void Set(Isolate* isolate, Local<S> handle);
 
  private:
-  static const int kInitialValue = -1;
-  int index_;
+  T* val_;
 };
 
 
@@ -7628,10 +7627,7 @@ class V8_EXPORT V8 {
                        WeakCallbackInfo<void>::Callback weak_callback);
   static void MakeWeak(internal::Object*** location_addr);
   static void* ClearWeak(internal::Object** location);
-  static void Eternalize(Isolate* isolate,
-                         Value* handle,
-                         int* index);
-  static Local<Value> GetEternal(Isolate* isolate, int index);
+  static Value* Eternalize(Isolate* isolate, Value* handle);
 
   static void RegisterExternallyReferencedObject(internal::Object** object,
                                                  internal::Isolate* isolate);
@@ -8596,12 +8592,15 @@ template<class T>
 template<class S>
 void Eternal<T>::Set(Isolate* isolate, Local<S> handle) {
   TYPE_CHECK(T, S);
-  V8::Eternalize(isolate, reinterpret_cast<Value*>(*handle), &this->index_);
+  val_ = reinterpret_cast<T*>(
+      V8::Eternalize(isolate, reinterpret_cast<Value*>(*handle)));
 }
 
 template <class T>
 Local<T> Eternal<T>::Get(Isolate* isolate) const {
-  return Local<T>(reinterpret_cast<T*>(*V8::GetEternal(isolate, index_)));
+  // The eternal handle will never go away, so as with the roots, we don't even
+  // need to open a handle.
+  return Local<T>(val_);
 }
 
 

deps/v8/src/api.cc

@@ -936,17 +936,13 @@ void V8::DisposeGlobal(i::Object** location) {
   i::GlobalHandles::Destroy(location);
 }
 
-
-void V8::Eternalize(Isolate* v8_isolate, Value* value, int* index) {
+Value* V8::Eternalize(Isolate* v8_isolate, Value* value) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   i::Object* object = *Utils::OpenHandle(value);
-  isolate->eternal_handles()->Create(isolate, object, index);
-}
-
-
-Local<Value> V8::GetEternal(Isolate* v8_isolate, int index) {
-  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
-  return Utils::ToLocal(isolate->eternal_handles()->Get(index));
+  int index = -1;
+  isolate->eternal_handles()->Create(isolate, object, &index);
+  return reinterpret_cast<Value*>(
+      isolate->eternal_handles()->Get(index).location());
 }