name of the top level "DEPS" file of a solution from within the .gclient spec.

This allows users to edit their gclient once and use a different (perhaps
experimental) DEPS file instead of the traditionally named "DEPS".

Review URL: http://codereview.chromium.org/203040

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26031 0039d316-1c4b-4281-b951-d872f2087c98