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This CL adds |device_heartbeat_settings| to the device policy proto.
We accidentally added |device_heartbeat_settings| with index 34 on
the server and |reboot_on_shutdown| with the same index on the client.
To resolve this conflict, one of the two settings needs to change
index. Since |device_heartbeat_settings| is not being read or written
by anyone yet, it is the one to move.

We must avoid such conflicts in the future by adding all policies on
the client-side first and syncing the resulting proto to the server.

In general, new policies should be added to the device policy proto
and policy_templates.json at the same time. As an emergency fix, this
CL gets a free pass for that rule as it is an emergency fix.

BUG=430908
TEST=None

Review URL: https://codereview.chromium.org/887423002

Cr-Commit-Position: refs/heads/master@{#314346}