Log Message:
Update chromium-bsu to 0.9.15.1.

Condensed changes:
* Translation updates
* Various code cleanup
* Fix movement using the diagonal keys on keypads
* Initialise the horizontal key movement speed properly
* Documentation improvements
* Add support for playing on a keyboard to the SDL backend.
* Add a new fourth level
* Various minor bugfixes
* Change the high scores file from a memory dump to plain text.
  Users with an existing high scores file in memory dump format will
  have their high scores file converted to plain text on startup.