[core] Prepare for OfflineFileSource

* Rename existing DefaultFileSource to OnlineFileSource
* Restore a DefaultFileSource that's a passthrough to OnlineFileSource

include/mbgl/storage/default_file_source.hpp

@@ -1,32 +1,25 @@
-#ifndef MBGL_STORAGE_DEFAULT_DEFAULT_FILE_SOURCE
-#define MBGL_STORAGE_DEFAULT_DEFAULT_FILE_SOURCE
+#ifndef MBGL_STORAGE_DEFAULT_FILE_SOURCE
+#define MBGL_STORAGE_DEFAULT_FILE_SOURCE
 
 #include <mbgl/storage/file_source.hpp>
-#include <mbgl/storage/file_cache.hpp>
 
 namespace mbgl {
 
-namespace util {
-template <typename T> class Thread;
-} // namespace util
+class FileCache;
 
 class DefaultFileSource : public FileSource {
 public:
-    DefaultFileSource(FileCache *cache, const std::string &root = "");
+    DefaultFileSource(FileCache*, const std::string& root = "");
     ~DefaultFileSource() override;
 
-    void setAccessToken(const std::string& t) { accessToken = t; }
-    std::string getAccessToken() const { return accessToken; }
+    void setAccessToken(const std::string&);
+    std::string getAccessToken() const;
 
     std::unique_ptr<FileRequest> request(const Resource&, Callback) override;
 
 private:
-    friend class DefaultFileRequest;
-    void cancel(const Resource&, FileRequest*);
-
     class Impl;
-    const std::unique_ptr<util::Thread<Impl>> thread;
-    std::string accessToken;
+    const std::unique_ptr<Impl> impl;
 };
 
 } // namespace mbgl