amp-google-document-embed doesn't load the embed document all the time

### Description

Using the `amp-google-document-embed` to embed a PDF document doesn't work all the time, sometimes the loader just hangs there.

### Reproduction Steps

Add the following code to an AMP page, and reload the page a few times.

```js

```

### Relevant Logs

_No response_

### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start with the amp-google-document-embed entry point and reproduce the issue using the supplied AMP markup and PDF URL, reloading the page several times. Trace the loading path to determine why the embed can hang; done means the PDF loads reliably across repeated reloads.

Written by the indexing model from the issue text.