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
const MediaPlayerInfo& video_type,
    const content::MediaPlayerId& id,
    content::WsObserver::MediaStoppedReason reason) {
  Emit("media-paused");
}
void Ws::DidChangeThemeColor() {
  auto theme_color = ontents()->GetThemeColor();
  if (theme_color) {
    Emit("did-change-theme-color", electron::ToRGBHex(theme_color.value()));
  } else {
    Emit("did-change-theme-color", nullptr);
  }
}
void Ws::OnInterfaceRequestFromFrame(
    content::RenderFrameHost* render_frame_host,
    const std::string& interface_name,
    mojo::ScopedMessagePipeHandle* interface_pipe) {
  registry_.TryBindInterface(interface_name, interface_pipe, render_frame_host);
}
void Ws::DidAcquireFullscreen(content::RenderFrameHost* rfh) {
  set_fullscreen_frame(rfh);
}
void Ws::DOMContentLoaded(
    content::RenderFrameHost* render_frame_host) {
  if (!render_frame_host->GetParent())
    Emit("dom-ready");
}
void Ws::DidFinishLoad(content::RenderFrameHost* render_frame_host,