Previously, 'gclient sync' would process "gn_args" settings in any
recursed DEPS files, potentially producing multiple output files or
conflicting output files, but 'gclient flatten' would only ever include
one set of "gn_args" settings in the flattened output, and only if they
occurred in the top-level DEPS file.

This makes 'gclient sync' and 'gclient flatten' more consistent by
restricting them both to a single instance of those setting, and
requiring those setting to be defined either in the top-level DEPS file,
or in a recursedeps file specificed by the new 'gclient_gn_args_from'
setting.

R=dpranke@google.com, ehmaldonado@google.com

Bug: 825063
Change-Id: If90d952e47367c50b36daade16a26b29aec0c9db
Reviewed-on: https://chromium-review.googlesource.com/1039870Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>