Add |device_heartbeat_settings| to device policy proto

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}

chrome/browser/chromeos/policy/proto/chrome_device_policy.proto

@@ -594,6 +594,17 @@ message RebootOnShutdownProto {
   optional bool reboot_on_shutdown = 1 [default = false];
 }
 
+// Settings that control whether a device would send heartbeat messages to GCM,
+// and how frequently to send these.
+message DeviceHeartbeatSettingsProto {
+  // Whether the device should send heartbeat messages. The default is false.
+  optional bool heartbeat_enabled = 1 [default = false];
+
+  // How frequently devices send heartbeats back to server. The unit is in
+  // milliseconds. The default is 2 minutes.
+  optional int64 heartbeat_frequency = 2 [default = 120000];
+}
+
 message ChromeDeviceSettingsProto {
   optional DevicePolicyRefreshRateProto device_policy_refresh_rate = 1;
   optional UserWhitelistProto user_whitelist = 2;
@@ -631,4 +642,5 @@ message ChromeDeviceSettingsProto {
   optional SystemSettingsProto system_settings = 32;
   optional SAMLSettingsProto saml_settings = 33;
   optional RebootOnShutdownProto reboot_on_shutdown = 34;
+  optional DeviceHeartbeatSettingsProto device_heartbeat_settings = 35;
 }