There is 2 changes while moving the code:
- Making externals handling an option.
- Not trapping EnvironmentError.

This code will be reused by the commit queue.

TEST=unit tests
BUG=none

Review URL: http://codereview.chromium.org/6250177

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@74181 0039d316-1c4b-4281-b951-d872f2087c98