* 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::WebLocalFrame* local_frame =
      web_frame_->FindFrameByName(blink::WebString::FromUTF8(name))
          ->ToWebLocalFrame();
  if (local_frame)
    return mate::CreateHandle(isolate(), new WebFrame(isolate(), local_frame))
        .ToV8();
  else
    return v8::Null(isolate());
}
v8::Local<v8::Value> WebFrame::FindFrameByRoutingId(int routing_id) const {
  content::RenderFrame* render_frame =
      content::RenderFrame::FromRoutingID(routing_id);
  blink::WebLocalFrame* local_frame = nullptr;
  if (render_frame)
    local_frame = render_frame->GetWebFrame();
  if (local_frame)
    return mate::CreateHandle(isolate(), new WebFrame(isolate(), local_frame))
        .ToV8();
  else
    return v8::Null(isolate());
}
v8::Local<v8::Value> WebFrame::RoutingId() const {
  int routing_id = content::RenderFrame::GetRoutingIdForWebFrame(web_frame_);
  return v8::Number::New(isolate(), routing_id);
}
void WebFrame::BuildPrototype(v8::Isolate* isolate,