[Fuchsia] Update webrunner to quit when sys::Runner is closed.

appmgr process normally creates a single sys::Runner connection for
each runner process and then closes it when it no longer needs the
runner. Updagted webrunner implementation to quit after sys::Runned
is destroyed.

Bug: 856416
Change-Id: I071b37093917a932a837eebfdf1f1a6e23e837de
Reviewed-on: https://chromium-review.googlesource.com/1134415
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574644}
3 files changed