We should use UserID object to identify users instead of username.

components/policy/core/common/cloud/component_cloud_policy_store.cc

Index: components/policy/core/common/cloud/component_cloud_policy_store.cc
diff --git a/components/policy/core/common/cloud/component_cloud_policy_store.cc b/components/policy/core/common/cloud/component_cloud_policy_store.cc
index dbcff3bdf2b4941d8519df4942da6d49a9affe82..257e7e689c126eec17a97e532e76e14e9fc4a936 100644
--- a/components/policy/core/common/cloud/component_cloud_policy_store.cc
+++ b/components/policy/core/common/cloud/component_cloud_policy_store.cc
@@ -66,7 +66,8 @@ ComponentCloudPolicyStore::ComponentCloudPolicyStore(
     Delegate* delegate,
     ResourceCache* cache)
     : delegate_(delegate),
-      cache_(cache) {
+      cache_(cache),
+      user_id_(std::string(), std::string()) {
   // Allow the store to be created on a different thread than the thread that
   // will end up using it.
   DetachFromThread();
@@ -107,12 +108,12 @@ const std::string& ComponentCloudPolicyStore::GetCachedHash(
   return it == cached_hashes_.end() ? base::EmptyString() : it->second;
 }
 
-void ComponentCloudPolicyStore::SetCredentials(const std::string& username,
+void ComponentCloudPolicyStore::SetCredentials(const user_manager::UserID& user_id,
                                                const std::string& dm_token) {
   DCHECK(CalledOnValidThread());
-  DCHECK(username_.empty() || username == username_);
+  DCHECK(user_id_.empty() || user_id == user_id_);
   DCHECK(dm_token_.empty() || dm_token == dm_token_);
-  username_ = username;
+  user_id_ = user_id_;
   dm_token_ = dm_token;
 }
 
@@ -273,13 +274,13 @@ bool ComponentCloudPolicyStore::ValidateProto(
     const std::string& settings_entity_id,
     em::ExternalPolicyData* payload,
     em::PolicyData* policy_data) {
-  if (username_.empty() || dm_token_.empty())
+  if (user_id_.empty() || dm_token_.empty())
     return false;
 
   scoped_ptr<ComponentCloudPolicyValidator> validator(
       ComponentCloudPolicyValidator::Create(
           proto.Pass(), scoped_refptr<base::SequencedTaskRunner>()));
-  validator->ValidateUsername(username_, true);
+  validator->ValidateUsername(user_id_, true);
   validator->ValidateDMToken(dm_token_,
                              ComponentCloudPolicyValidator::DM_TOKEN_REQUIRED);
   if (!policy_type.empty())