Merge pull request #13821 (update chromium)

This is just a minor upgrade, even though the commit message says it's
to major version 50. However, the CVEs listed there are for real, see
the following announcement:

http://googlechromereleases.blogspot.de/2016/03/stable-channel-update_8.html

The summary of updated packages:

stable: 49.0.2623.75 -> 49.0.2623.87
beta:   49.0.2623.75 -> 50.0.2661.26
dev:    50.0.2661.11 -> 50.0.2661.18

I've also added two commits, fixing the chdir() in the updater and
shutting up Python precompilation errors during the preBuild phase.

Tested on my Hydra at:

https://headcounter.org/hydra/eval/312166