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This migrates the start_host tool to access the new Directory API
using gRPC.

Bug: 968326
Change-Id: Iac944cfe1d072b977f0d71feb7ad3e30f168e385
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636671
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664645}