Implement parsing of Chromium .DEPS files.

This is necessary to be able to checkout a chromium
release branch. Chromium release branches do not
make use of git submodule.
Instead the dependencies or submodules are specified
in a .DEPS.git file.
Given some python scopes and callbacks for resolving
variables, such a file can be invoked like a python
script. The dependencies will then be exported
into the provided scopes and we can use that
information to initialize git submodules.

Change-Id: Ibab09b1077720b73f89efff16d38b308f31cf2ab
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>