gclient flatten: also preserve variables in deps_os entries

This is the same change as in https://chromium-review.googlesource.com/c/586594
applied to _DepsOsToLines.

Bug: 570091
Change-Id: I06a4dc3a9b6dbb001a15b1d7b88fef12cf6c6aaa
Reviewed-on: https://chromium-review.googlesource.com/609980
Reviewed-by: Emma Söderberg <emso@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
diff --git a/gclient.py b/gclient.py
index 99f50a0..9e3ed81 100755
--- a/gclient.py
+++ b/gclient.py
@@ -1890,7 +1890,7 @@
       s.extend([
           '    # %s' % dep.hierarchy(include_url=False),
           '    "%s": {' % (name,),
-          '      "url": "%s",' % (dep.url,),
+          '      "url": "%s",' % (dep.raw_url,),
       ] + condition_part + [
           '    },',
           '',