[core] Move OnlineFileSource to platform

Preparation for OfflineFileSource which depends on SQLite

platform/default/default_file_source.cpp

@@ -0,0 +1,34 @@
+#include <mbgl/storage/default_file_source.hpp>
+
+#include "online_file_source.hpp"
+
+namespace mbgl {
+
+class DefaultFileSource::Impl {
+public:
+    Impl(FileCache* cache, const std::string& root)
+        : onlineFileSource(cache, root) {
+    }
+
+    OnlineFileSource onlineFileSource;
+};
+
+DefaultFileSource::DefaultFileSource(FileCache* cache, const std::string& root)
+    : impl(std::make_unique<DefaultFileSource::Impl>(cache, root)) {
+}
+
+DefaultFileSource::~DefaultFileSource() = default;
+
+void DefaultFileSource::setAccessToken(const std::string& accessToken) {
+    impl->onlineFileSource.setAccessToken(accessToken);
+}
+
+std::string DefaultFileSource::getAccessToken() const {
+    return impl->onlineFileSource.getAccessToken();
+}
+
+std::unique_ptr<FileRequest> DefaultFileSource::request(const Resource& resource, Callback callback) {
+    return impl->onlineFileSource.request(resource, callback);
+}
+
+} // namespace mbgl