Intermittent crash in wpt reftests

[jdm]

This may or may not be the same underlying cause as #26313.

  ▶ CRASH [expected FAIL] /css/CSS2/floats-clear/clear-applies-to-015.xht
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  └ Shutting down the Constellation after generating an output file or exit flag specified

  ▶ CRASH [expected PASS] /css/CSS2/normal-flow/width-100.xht
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  └ Shutting down the Constellation after generating an output file or exit flag specified

[jdm]

 ▶ CRASH [expected FAIL] /css/css-backgrounds/background-size/vector/wide--12px-auto--percent-width-percent-height.html
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  │ Shutting down the Constellation after generating an output file or exit flag specified
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  └ Shutting down the Constellation after generating an output file or exit flag specified

[jdm]

  ▶ CRASH [expected FAIL] /css/CSS2/selectors/first-letter-punctuation-272.xht
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  └ Shutting down the Constellation after generating an output file or exit flag specified

[CYBAI]

Also observed in layout-2020

  ▶ CRASH [expected PASS] /css/CSS2/normal-flow/height-112.xht
  │ 
  │ error: XDG_RUNTIME_DIR not set in the environment.
  │ libEGL warning: No hardware driver found, falling back to software rendering
  └ Shutting down the Constellation after generating an output file or exit flag specified