login: Wait for all descendants in browser process group Since kernel 3.4, it's been possible for a process to wait on all descendants, not just direct children, given the appropriate prctl(). The session_manager has always wanted to do this, so let's do it! BUG=chromium:436549 TEST=bvt-inline, unittests Change-Id: I7d2048cd11762d077f346c0d6e473ba9307c779d Previous-Reviewed-on: https://chromium-review.googlesource.com/252803 Trybot-Ready: Chris Masone <[email protected]> (cherry picked from commit 1ebe027077ef6a2e0c090143bc5179a7f3531e62) Reviewed-on: https://chromium-review.googlesource.com/253791 Reviewed-by: Yuly Novikov <[email protected]> Commit-Queue: Yuly Novikov <[email protected]> Tested-by: Yuly Novikov <[email protected]>