In iossim, use the user's home directory if no override is given.

If no override is given, simHomePath needs to be initialized or iossim
won't know where to find files (e.g. the simulator's sandboxed
system.log)

BUG=None

Review URL: https://codereview.chromium.org/398133002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@283643 4ff67af0-8c30-449e-8e8b-ad334ec8d88c