* 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
script.Get("url", &url);
    script.Get("startLine", &start_line);
    if (!script.Get("code", &code)) {
      args->ThrowError("Invalid 'code'");
      return;
    }
    sources.emplace_back(
        blink::WebScriptSource(blink::WebString::FromUTF16(code),
                               blink::WebURL(GURL(url)), start_line));
  }
  bool has_user_gesture = false;
  args->GetNext(&has_user_gesture);
  blink::WebLocalFrame::ScriptExecutionType scriptExecutionType =
      blink::WebLocalFrame::kSynchronous;
  args->GetNext(&scriptExecutionType);
  ScriptExecutionCallback::CompletionCallback completion_callback;
  args->GetNext(&completion_callback);
  std::unique_ptr<blink::WebScriptExecutionCallback> callback(
      new ScriptExecutionCallback(completion_callback));
  web_frame_->RequestExecuteScriptInIsolatedWorld(
      world_id, &sources.front(), sources.size(), has_user_gesture,
      scriptExecutionType, callback.release());
}
void WebFrame::SetIsolatedWorldSecurityOrigin(int world_id,
                                              const std::string& origin_url) {
  web_frame_->SetIsolatedWorldSecurityOrigin(
      world_id, blink::WebSecurityOrigin::CreateFromString(
                    blink::WebString::FromUTF8(origin_url)));
}
void WebFrame::SetIsolatedWorldContentSecurityPolicy(