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):

if (properties.isInternalFont) {
    return;
  }
  if (builtInEncoding === properties.defaultEncoding) {
    return; 
  }
  if (properties.toUnicode instanceof IdentityToUnicodeMap) {
    return;
  }
  const toUnicode = [],
    glyphsUnicodeMap = getGlyphsUnicode();
  for (const charCode in builtInEncoding) {
    if (properties.hasIncludedToUnicodeMap) {
      if (properties.toUnicode.has(charCode)) {
        continue; 
      }
    } else {
      if (
        properties.hasEncoding &&
        properties.differences[charCode] !== undefined
      ) {
        continue; 
      }
    }
    const glyphName = builtInEncoding[charCode];
    const unicode = getUnicodeForGlyph(glyphName, glyphsUnicodeMap);
    if (unicode !== -1) {