This ability is useful for versions of Chromium (i.e. Android) which want to
cross-compile to another platform that has a fixed set of custom dependencies.

Review URL: http://codereview.chromium.org/10127004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@134280 0039d316-1c4b-4281-b951-d872f2087c98