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Integrating the launch watcher into the context manager so that the
startup process can await on borealis starting.

Tests: Built and ran tests, tested on DUT.
Bug: b/171662734
Change-Id: I917440d9c190242bc7d4e28120e716702496f60c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497344
Commit-Queue: Daniel Ng <danielng@google.com>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#820648}