This will make it easier to programmatically determine where a flattened
entry originated, which is needed for recursively branching and
unpinning DEPS files.

BUG=825063
R=dpranke@google.com

Change-Id: Id280c0b0a95b8664602e0ec4513722fe4d6d1ebf
Reviewed-on: https://chromium-review.googlesource.com/977326Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>