gclient flatten: parse DEPS file for deps_os recursedeps

Bug: 570091
Change-Id: I773b74b042233efa2a525f5f47e920468b7fea4a
Reviewed-on: https://chromium-review.googlesource.com/618930
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
diff --git a/gclient.py b/gclient.py
index f0a1303..72b0019 100755
--- a/gclient.py
+++ b/gclient.py
@@ -1799,6 +1799,9 @@
     Arguments:
       dep (Dependency): dependency to process
     """
+    if not dep.deps_parsed:
+      dep.ParseDepsFile()
+
     self._allowed_hosts.update(dep.allowed_hosts)
 
     for key, value in dep.get_vars().iteritems():