* 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 (icon->image().IsEmpty()) {
    args->ThrowError("Must specify non-empty 'icon' option");
    return;
  }
#endif
  
  if (!files.empty()) {
    base::MessageLoop::ScopedNestableTaskAllower allow(
        base::MessageLoop::current());
    DragFileItems(files, icon->image(), web_contents()->GetNativeView());
  } else {
    args->ThrowError("Must specify either 'file' or 'files' option");
  }
}
void WebContents::CapturePage(mate::Arguments* args) {
  gfx::Rect rect;
  base::Callback<void(const gfx::Image&)> callback;
  if (!(args->Length() == 1 && args->GetNext(&callback)) &&
      !(args->Length() == 2 && args->GetNext(&rect)
                            && args->GetNext(&callback))) {
    args->ThrowError();
    return;
  }
  const auto view = web_contents()->GetRenderWidgetHostView();
  if (!view) {
    callback.Run(gfx::Image());
    return;
  }
  
  const gfx::Size view_size = rect.IsEmpty() ? view->GetViewBounds().size() :