Add back the constant SHUTDOWN_TIMEOUT which was accidentally removed

The constant SHUTDOWN_TIMEOUT was accidentally removed in:
  https://chromium-review.googlesource.com/#/c/56348/

However, this constant is still used by test_wait_for_shutdown().
Add it back.

BUG=chromium:343125
TEST=None, trivial change.

Change-Id: Ie4708a47cd54592fbacccd871b93139fd811a79f
Reviewed-on: https://chromium-review.googlesource.com/186346
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
1 file changed