Attach (recommended) or Link to PDF file here:
[chinese.pdf](https://github.com/mozilla/pdf.js/files/6404389/download.1.pdf)

Configuration:
- Web browser and its version:90.0.4430.93
- Operating system and its version: mac 10.15.5
- PDF.js version: 2.9.146
- Is a browser extension:no

Steps to reproduce the problem:
1. The Chinese in the attachment cannot be rendered

Brower ✅
![image](https://user-images.githubusercontent.com/13477114/116678074-81782a00-a9db-11eb-8719-74f48fcebc2b.png)

PDF.js ❌ [https://mozilla.github.io/pdf.js/web/viewer.html](https://mozilla.github.io/pdf.js/web/viewer.html)
![image](https://user-images.githubusercontent.com/13477114/116678212-a371ac80-a9db-11eb-847f-dd1ad7752c13.png)


What is the expected behavior? (add screenshot)
**The Chinese character can be rendered**
What went wrong? (add screenshot)
**All of the Chinese character cannot be rendered**
Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

}
    
    return (this._charsCache[chars] = glyphs);
  }
  /**
   * Chars can have different sizes (depends on the encoding).
   * @param {String} a string encoded with font encoding.
   * @returns {Array<Array<number>>} the positions of each char in the string.
   */
  getCharPositions(chars) {
    
    
    const positions = [];
    if (this.cMap) {
      const c = Object.create(null);
      let i = 0;
      while (i < chars.length) {
        this.cMap.readCharCode(chars, i, c);
        const length = c.length;
        positions.push([i, i + length]);
        i += length;
      }
    } else {
      for (let i = 0, ii = chars.length; i < ii; ++i) {
        positions.push([i, i + 1]);
      }
    }
    return positions;
  }
  get glyphCacheValues() {