We need to pass OS info to recursively called _flatten_dep.
Regular deps entries in OS-specific DEPS file recursed into
need to be marked as OS-specific in the flattened file.

Bug: 570091
Change-Id: If3055b84143d8a52d10d8753113893b5054b4d07
Reviewed-on: https://chromium-review.googlesource.com/621046Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>