* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
const base::Callback<void(bool)>& callback) {
  auto context = GetServiceWorkerContext(web_contents());
  if (!context)
    return;
  struct WrappedCallback {
    base::Callback<void(bool)> callback_;
    explicit WrappedCallback(const base::Callback<void(bool)>& callback)
        : callback_(callback) {}
    void Run(content::ServiceWorkerCapability capability) {
      callback_.Run(capability !=
                    content::ServiceWorkerCapability::NO_SERVICE_WORKER);
      delete this;
    }
  };
  auto wrapped_callback = new WrappedCallback(callback);
  context->CheckHasServiceWorker(
      web_contents()->GetLastCommittedURL(), GURL::EmptyGURL(),
      base::Bind(&WrappedCallback::Run, base::Unretained(wrapped_callback)));
}
void WebContents::UnregisterServiceWorker(
    const base::Callback<void(bool)>& callback) {
  auto context = GetServiceWorkerContext(web_contents());
  if (!context)
    return;
  context->UnregisterServiceWorker(web_contents()->GetLastCommittedURL(),
                                   callback);
}
void WebContents::SetIgnoreMenuShortcuts(bool ignore) {