Expose signing key from cloud policy stores

chrome/browser/chromeos/policy/device_cloud_policy_store_chromeos.cc

Index: chrome/browser/chromeos/policy/device_cloud_policy_store_chromeos.cc
diff --git a/chrome/browser/chromeos/policy/device_cloud_policy_store_chromeos.cc b/chrome/browser/chromeos/policy/device_cloud_policy_store_chromeos.cc
index 5317131bef6b17561c5f49c484cfb9bc333e5971..46f96083994df114f85a370f2148a843393417cf 100644
--- a/chrome/browser/chromeos/policy/device_cloud_policy_store_chromeos.cc
+++ b/chrome/browser/chromeos/policy/device_cloud_policy_store_chromeos.cc
@@ -164,6 +164,10 @@ void DeviceCloudPolicyStoreChromeOS::UpdateFromService() {
     }
     policy_map_.Swap(&new_policy_map);
 
+    scoped_refptr<ownership::PublicKey> key =
+        device_settings_service_->GetPublicKey();
+    policy_signature_public_key_ = key ? key->as_string() : std::string();
+
     NotifyStoreLoaded();
     return;
   }