* 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
if (embedder) {
    NativeWindow* owner_window = nullptr;
    auto relay = NativeWindowRelay::FromWebContents(embedder->web_contents());
    if (relay) {
      owner_window = relay->window.get();
    }
    if (owner_window)
      SetOwnerWindow(owner_window);
    content::RenderWidgetHostView* rwhv =
        web_contents()->GetRenderWidgetHostView();
    if (rwhv) {
      rwhv->Hide();
      rwhv->Show();
    }
  }
}
v8::Local<v8::Value> WebContents::GetNativeView() const {
  gfx::NativeView ptr = web_contents()->GetNativeView();
  auto buffer = node::Buffer::Copy(
      isolate(), reinterpret_cast<char*>(&ptr), sizeof(gfx::NativeView));
  if (buffer.IsEmpty())
    return v8::Null(isolate());
  else
    return buffer.ToLocalChecked();
}
v8::Local<v8::Value> WebContents::DevToolsWebContents(v8::Isolate* isolate) {