[CrOS Tether]: Add LocalDeviceDataProvider, which fetches device data for the local device.

components/cryptauth/cryptauth_enrollment_manager.h

Index: components/cryptauth/cryptauth_enrollment_manager.h
diff --git a/components/cryptauth/cryptauth_enrollment_manager.h b/components/cryptauth/cryptauth_enrollment_manager.h
index 79baf62f65856b284b57e7aa9db4add0eed8d658..55dca06b10752197e54f7bca815a4dd6952018a1 100644
--- a/components/cryptauth/cryptauth_enrollment_manager.h
+++ b/components/cryptauth/cryptauth_enrollment_manager.h
@@ -112,8 +112,8 @@ class CryptAuthEnrollmentManager : public SyncScheduler::Delegate,
   // been completed, then an empty string will be returned.
   // Note: These keys are really serialized protocol buffer messages, and should
   // only be used by passing to SecureMessageDelegate.
-  std::string GetUserPublicKey();
-  std::string GetUserPrivateKey();
+  std::string GetUserPublicKey() const;
+  std::string GetUserPrivateKey() const;
 
  protected:
   // Creates a new SyncScheduler instance. Exposed for testing.