Add a run_testserver executable to make it easy to run the testserver

standalone.

Example:
$ testserver --doc-root=net/data/url_request_unittest --http

Also supports --https and --ftp.

R=eroman
BUG=none
TEST=none

Review URL: http://codereview.chromium.org/2901006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51993 0039d316-1c4b-4281-b951-d872f2087c98