cbuildbot/stages: Serialize DebugSymbols to run after UnitTest

DebugSymbols will now wait for UnitTest to finish before
starting. The tar operation in DebugSymbols was erroring
out because UnitTest was writing into the folder being
tar-ed.

BUG=chromium:547055
TEST=cbuildbot/run_tests

Change-Id: I02d2b9faeeb5dd304a14508e1d58529dab858fc5
Reviewed-on: https://chromium-review.googlesource.com/1145825
Commit-Ready: Dhanya Ganesh <dhanyaganesh@google.com>
Tested-by: Dhanya Ganesh <dhanyaganesh@google.com>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
(cherry picked from commit 09f0990a2db0cf45d076a0c6d99a88853b258a10)
Reviewed-on: https://chromium-review.googlesource.com/1180404
Commit-Queue: Dhanya Ganesh <dhanyaganesh@chromium.org>
Tested-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Trybot-Ready: Dhanya Ganesh <dhanyaganesh@chromium.org>
3 files changed