Use gclient setdep/getdep instead of evaluating the contents of the
DEPS file, so we don't have to deal with builtin_variables.

Bug: 906114
Change-Id: I2082d4a3feb84d15c251b7d99056fd4c6f925453
Reviewed-on: https://chromium-review.googlesource.com/c/1347450Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>