Refactor configuration of sandboxes - first steps

See
https://docs.google.com/document/d/1H-hCsIcMsAEP0fWHimbuiNA-Hc9eXEmR94eb-2RQAhA/edit?usp=sharing
for background.

This moves all process type dependent decisions on how to create
Linux processes (not how to sandbox them once created, not Android)
into the launch delegates and makes the arguments to the
ChildProcessLauncher constructor and
BrowserChildProcessHostImpl::Launch OS independent.

BUG=none

Review URL: https://codereview.chromium.org/177863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256802 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256802