components: Add out-of-line copy ctors for complex classes.

components/proximity_auth/remote_device.cc

Index: components/proximity_auth/remote_device.cc
diff --git a/components/proximity_auth/remote_device.cc b/components/proximity_auth/remote_device.cc
index cabe7149dadf5ba183a9b7b2e8c8a3eef4fd69d5..cecf0239b8e1faa50b620eb280d2d23cbfd45db7 100644
--- a/components/proximity_auth/remote_device.cc
+++ b/components/proximity_auth/remote_device.cc
@@ -23,6 +23,8 @@ RemoteDevice::RemoteDevice(const std::string& user_id,
       persistent_symmetric_key(persistent_symmetric_key),
       sign_in_challenge(sign_in_challenge) {}
 
+RemoteDevice::RemoteDevice(const RemoteDevice& other) = default;
+
 RemoteDevice::~RemoteDevice() {}
 
 }  // namespace