Pulls pexe and tar files for PNaCl kernel
Pulls pexe and tar files for PNaCl kernel

Adds a step to the install_chrome.py script, to pull in files needed for the PNaCl kernel.  Currently these are being downloaded from a temporary location, but in the future they will be downloaded from the naclports continuous buidler.

jupyter/colaboratory,wernight/colaboratory,wernight/colaboratory,wernight/colaboratory,jupyter/colaboratory,jupyter/colaboratory