WebPDF exporter hangs and cpu usage spikes to 100% if DISPLAY variable is set.

[felder]

If chosen via the UI or run via the command line, jupyter nbconvert --to webpdf will hang and CPU usage for chrome will spike to 100% if the DISPLAY environment variable is set.

From the cli, if I run unset DISPLAY prior to trying jupyter nbconvert --to webpdf everything works fine.

How to reproduce:

1. create a new blank notebook named blank.ipynb (or choose an existing notebook)
2. open a terminal on an OS that supports a DISPLAY environment variable
3. run export DISPLAY=":1.0"
4. run jupyter nbconvert --to webpdf blank.ipynb