Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Firefox 88.0.1
- Operating system and its version: Windows 10.0.19043.985]
- PDF.js version: (commit 706c27b9d4b3b9ff6fc3de82bcf71a0610604a1c) (also tested pr/13473 at 296bbdc4)
- Is a browser extension: N/A

Steps to reproduce the problem:
1. Enable experimental XFA
2. Open PDF
[dir2020-02_form.pdf](https://github.com/mozilla/pdf.js/files/6571084/dir2020-02_form.pdf)

What is the expected behavior? (add screenshot)
Margins of `subforms` (e.g. *General_Election_Information*, *General_Information_Other*) are honored.
![image](https://user-images.githubusercontent.com/7104454/120212605-32066300-c200-11eb-9c66-f2d05c9451ec.png)
What went wrong? (add screenshot)
Margins are not honored. 
![image](https://user-images.githubusercontent.com/7104454/120212535-1ac77580-c200-11eb-8332-9924602ef991.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

let marginV = 0;
    if (this.margin) {
      marginH = this.margin.leftInset + this.margin.rightInset;
      marginV = this.margin.topInset + this.margin.bottomInset;
    }
    if (this.w === "") {
      style.width = measureToString(this[$extra].width + marginH);
    }
    if (this.h === "") {
      style.height = measureToString(this[$extra].height + marginV);
    }
    let html = {
      name: "div",
      attributes,
      children,
    };
    html = addExtraDivForBorder(html);
    let bbox;
    if (this.w !== "" && this.h !== "") {
      bbox = [this.x, this.y, this.w, this.h];
    } else {
      const width = this.w === "" ? marginH + this[$extra].width : this.w;
      const height = this.h === "" ? marginV + this[$extra].height : this.h;
      bbox = [this.x, this.y, width, height];
    }
    delete this[$extra];
    return HTMLResult.success(html, bbox);
  }
}
class Execute extends XFAObject {