Revert of Remove the weak list of array buffers

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index c42cab62d55c64735efb722c85f046ab8a2830df..de0b0076e0901093d5c8dab59880495ad567342e 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -6254,12 +6254,9 @@
 
 v8::ArrayBuffer::Contents v8::ArrayBuffer::Externalize() {
   i::Handle<i::JSArrayBuffer> self = Utils::OpenHandle(this);
-  i::Isolate* isolate = self->GetIsolate();
   Utils::ApiCheck(!self->is_external(), "v8::ArrayBuffer::Externalize",
                   "ArrayBuffer already externalized");
   self->set_is_external(true);
-  isolate->heap()->UnregisterArrayBuffer(self->backing_store());
-
   return GetContents();
 }