* Output of `node_modules/.bin/electron --version`: v3.0.5
* Operating System (Platform and Version): macOS 10.14

**Expected Behavior**
When webPreferences.sandbox is set to true and webFrame is using a provider for spell checking, the memory must be released after page reload.

**Actual behavior**
The memory isn't released and caused memory leak

**To Reproduce**

- Clone https://github.com/vrbsm/example-memory-leak-electron
- Run npm run start
Memory usage of electron process will be increasing in task manager after reloading the page several times

**Screenshots**
![test-memory-leak-01](https://user-images.githubusercontent.com/9887288/47675838-c2d4b800-db99-11e8-97e8-b5b27bcab929.gif)



atom/renderer/api/atom_api_web_frame.cc
blink::WebFrame* frame = web_frame_->Opener();
  if (frame && frame->IsWebLocalFrame())
    return mate::CreateHandle(isolate(),
                              new WebFrame(isolate(), frame->ToWebLocalFrame()))
        .ToV8();
  else
    return v8::Null(isolate());
}
v8::Local<v8::Value> WebFrame::Parent() const {
  blink::WebFrame* frame = web_frame_->Parent();
  if (frame && frame->IsWebLocalFrame())
    return mate::CreateHandle(isolate(),
                              new WebFrame(isolate(), frame->ToWebLocalFrame()))
        .ToV8();
  else
    return v8::Null(isolate());
}
v8::Local<v8::Value> WebFrame::Top() const {
  blink::WebFrame* frame = web_frame_->Top();
  if (frame && frame->IsWebLocalFrame())
    return mate::CreateHandle(isolate(),
                              new WebFrame(isolate(), frame->ToWebLocalFrame()))
        .ToV8();
  else
    return v8::Null(isolate());
}
v8::Local<v8::Value> WebFrame::FirstChild() const {