It's possible to tell gclient to use a different
"deps" file from the default DEPS through the "deps_file"
variable in the .gclient file.

If this file is missing, fallback to DEPS (the
default).

BUG=390700

Review URL: https://codereview.chromium.org/368713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@280921 0039d316-1c4b-4281-b951-d872f2087c98