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This CL introduces a lifecycle statemachine to AgentSchedulingGroupHost.
The `AgentSchedulingGroupHost::state_` makes it explicit which lifestage
the AgentSchedulingGroupHost is currently at, and checks if each method
is called on one of the assumed states.

This is a preparatory work to introducing AgentSchedulingGroupHost
shutdown, which will add more states to the state machine.

Bug: 1141459
Change-Id: Ic8c4f4f85322b52612510b3354d5ae6828311263
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2492281
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Auto-Submit: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Reviewed-by: Tal Pressman <talp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826664}