Electron Version: 7.1.1
Operating System: Windows 10 (1803)

If I set the 'margins' option when call webContents.print method,
The app will crash with no error message.

```js
let { remote } = require("electron");
let webContents = remote.getCurrentWebContents();
webContents.print(
  {
    margins: {
      marginType: "custom",
      top: 30,
      bottom: 0,
      left: 0,
      right: 0
    }
  },
  (success, errorType) => {
    if (!success) console.log(errorType);
  }
);
```

shell/browser/api/electron_api_web_contents.cc
}
void Ws::DidStartNavigation(
    content::NavigationHandle* navigation_handle) {
  EmitNavigationEvent("did-start-navigation", navigation_handle);
}
void Ws::DidRedirectNavigation(
    content::NavigationHandle* navigation_handle) {
  EmitNavigationEvent("did-redirect-navigation", navigation_handle);
}
void Ws::DidFinishNavigation(
    content::NavigationHandle* navigation_handle) {
  if (!navigation_handle->HasCommitted())
    return;
  bool is_main_frame = navigation_handle->IsInMainFrame();
  content::RenderFrameHost* frame_host =
      navigation_handle->GetRenderFrameHost();
  int frame_process_id = -1, frame_routing_id = -1;
  if (frame_host) {
    frame_process_id = frame_host->GetProcess()->GetID();
    frame_routing_id = frame_host->GetRoutingID();
  }
  if (!navigation_handle->IsErrorPage()) {
    // FIXME: All the Emit() calls below could potentially result in |this|
    // being destroyed (by JS listening for the event and calling
    // ws.destroy()).
    auto url = navigation_handle->GetURL();
    bool is_same_document = navigation_handle->IsSameDocument();
    if (is_same_document) {
      Emit("did-navigate-in-page", url, is_main_frame, frame_process_id,
           frame_routing_id);