Improvements to automate.py:
- Allow specification of the depot_tools directory via a "depot-tools" flag (issue #592).
- Add ninja build support via a "ninja-build" flag (issue #922).
- Allow relative paths for download directories (issue #942).
- Add the ability to print commands without executing them via a "dry-run" flag.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1194 5089003a-bbd8-11dd-ad1f-f1f9622dbc98