Use CompletionOnceCallback in ServerSocket.

Bug: 807724
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Idb037fd78ab3f092af08bef665c4f65c786cdd53
Reviewed-on: https://chromium-review.googlesource.com/1100900
Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575237}
14 files changed