Previously, gclient would attempt to write an args file after
a dependency was checked out, but before any sub-dependencies
had been checked out. If the args file path pointed at something
inside a sub-dependency, this wouldn't work, because the directory
might not yet exist. This most obviously happened for buildspec
clobber builds.

The fix is to wait until after the sub-dependencies have been
checked out to write the file.

R=phajdan.jr@chromium.org, mmoss@chromium.org
BUG=773933

Change-Id: I0cf4564204f7dabd9f843dc7904db7050fcc0d23
Reviewed-on: https://chromium-review.googlesource.com/714644Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>