[Fuchsia] Address CR comments in sys::Runner implementation

When addressing comments on https://crrev.com//c//1114377 I hit CQ+1
prematurely and landed the change without my changes that addresed
review comments. Landing them separately.

Bug: 856416
Change-Id: I1b691d824f381e9ec0a75ff7ce90b2f45171c13a
Reviewed-on: https://chromium-review.googlesource.com/1133968
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574598}
6 files changed