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
if (debugger_.IsEmpty()) {
    auto handle = electron::api::Debugger::Create(isolate, ontents());
    debugger_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, debugger_);
}
void Ws::GrantOriginAccess(const GURL& url) {
  content::ChildProcessSecurityPolicy::GetInstance()->GrantCommitOrigin(
      ontents()->GetMainFrame()->GetProcess()->GetID(),
      url::Origin::Create(url));
}
v8::Local<v8::Promise> Ws::TakeHeapSnapshot(
    const base::FilePath& file_path) {
  gin_helper::Promise<void> promise(isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
  base::ThreadRestrictions::ScopedAllowIO allow_io;
  base::File file(file_path,
                  base::File::FLAG_CREATE_ALWAYS | base::File::FLAG_WRITE);
  if (!file.IsValid()) {
    promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
  auto* frame_host = ontents()->GetMainFrame();
  if (!frame_host) {
    promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;