Apply suggestions from code review

Author: jasnell

src/util.cc

@@ -736,9 +736,8 @@ Local<String> UnionBytes::ToStringChecked(Isolate* isolate) const {
 }
 
 RAIIIsolateWithoutEntering::RAIIIsolateWithoutEntering(const SnapshotData* data)
-    : allocator_{ArrayBuffer::Allocator::NewDefaultAllocator()},
-      isolate_group_(GetOrCreateIsolateGroup()) {
-  isolate_ = Isolate::Allocate(isolate_group_);
+    : allocator_{ArrayBuffer::Allocator::NewDefaultAllocator()} {
+  isolate_ = Isolate::Allocate();
   CHECK_NOT_NULL(isolate_);
   per_process::v8_platform.Platform()->RegisterIsolate(isolate_,
                                                        uv_default_loop());

src/util.h

@@ -990,7 +990,6 @@ class RAIIIsolateWithoutEntering {
 
  private:
   std::unique_ptr<v8::ArrayBuffer::Allocator> allocator_;
-  v8::IsolateGroup isolate_group_;
   v8::Isolate* isolate_;
 };