[core] Use const& for the OfflineRegion object

It is used as const and also fixes a build issue on iOS:

```
Non-const lvalue reference to type ‘mbgl::OfflineRegion’ cannot bind
to a temporary of type ‘typename remove_reference<OfflineRegion &>::type’
(aka ‘mbgl::OfflineRegion’)
```

1 files changed, 7 insertions, 5 deletions

diff --git a/platform/default/src/mbgl/storage/database_file_source.cpp b/platform/default/src/mbgl/storage/database_file_source.cpp
index 4dff99b5b8..50c444274e 100644
--- a/platform/default/src/mbgl/storage/database_file_source.cpp
+++ b/platform/default/src/mbgl/storage/database_file_source.cpp
@@ -248,26 +248,28 @@ void DatabaseFileSource::updateOfflineMetadata(
     impl->actor().invoke(&DatabaseFileSourceThread::updateMetadata, regionID, metadata, std::move(callback));
 }
 
-void DatabaseFileSource::deleteOfflineRegion(OfflineRegion& region, std::function<void(std::exception_ptr)> callback) {
+void DatabaseFileSource::deleteOfflineRegion(const OfflineRegion& region,
+                                             std::function<void(std::exception_ptr)> callback) {
     impl->actor().invoke(&DatabaseFileSourceThread::deleteRegion, region, std::move(callback));
 }
 
-void DatabaseFileSource::invalidateOfflineRegion(OfflineRegion& region,
+void DatabaseFileSource::invalidateOfflineRegion(const OfflineRegion& region,
                                                  std::function<void(std::exception_ptr)> callback) {
     impl->actor().invoke(&DatabaseFileSourceThread::invalidateRegion, region.getID(), std::move(callback));
 }
 
-void DatabaseFileSource::setOfflineRegionObserver(OfflineRegion& region,
+void DatabaseFileSource::setOfflineRegionObserver(const OfflineRegion& region,
                                                   std::unique_ptr<OfflineRegionObserver> observer) {
     impl->actor().invoke(&DatabaseFileSourceThread::setRegionObserver, region.getID(), std::move(observer));
 }
 
-void DatabaseFileSource::setOfflineRegionDownloadState(OfflineRegion& region, OfflineRegionDownloadState state) {
+void DatabaseFileSource::setOfflineRegionDownloadState(const OfflineRegion& region, OfflineRegionDownloadState state) {
     impl->actor().invoke(&DatabaseFileSourceThread::setRegionDownloadState, region.getID(), state);
 }
 
 void DatabaseFileSource::getOfflineRegionStatus(
-    OfflineRegion& region, std::function<void(expected<OfflineRegionStatus, std::exception_ptr>)> callback) const {
+    const OfflineRegion& region,
+    std::function<void(expected<OfflineRegionStatus, std::exception_ptr>)> callback) const {
     impl->actor().invoke(&DatabaseFileSourceThread::getRegionStatus, region.getID(), std::move(callback));
 }