* 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.h
#ifndef ATOM_RENDERER_API_ATOM_API_WEB_FRAME_H_
#define ATOM_RENDERER_API_ATOM_API_WEB_FRAME_H_
#include <memory>
#include <string>
#include <vector>
#include "native_mate/handle.h"
#include "native_mate/wrappable.h"
#include "third_party/blink/public/platform/web_cache.h"
namespace blink {
class WebLocalFrame;
}
namespace mate {
class Dictionary;
class Arguments;
}  
namespace atom {
namespace api {
class WebFrame : public mate::Wrappable<WebFrame> {