[core] Moving caching logic to DefaultFileSource

This results in OnlineFileSource containing precisely the logic we want for reuse by OfflineFileSource, and no more.

include/mbgl/storage/default_file_source.hpp

@@ -5,6 +5,8 @@
 
 namespace mbgl {
 
+class SQLiteCache;
+
 class DefaultFileSource : public FileSource {
 public:
     DefaultFileSource(const std::string& cachePath, const std::string& assetRoot);
@@ -18,8 +20,13 @@ public:
 
     std::unique_ptr<FileRequest> request(const Resource&, Callback) override;
 
+    // For testing purposes only.
+    SQLiteCache& getCache();
+
 private:
     class Impl;
+    friend class DefaultFileRequest;
+
     const std::unique_ptr<Impl> impl;
 };
 

include/mbgl/storage/online_file_source.hpp

@@ -5,15 +5,13 @@
 
 namespace mbgl {
 
-class SQLiteCache;
-
 namespace util {
 template <typename T> class Thread;
 } // namespace util
 
 class OnlineFileSource : public FileSource {
 public:
-    OnlineFileSource(SQLiteCache*);
+    OnlineFileSource();
     ~OnlineFileSource() override;
 
     void setAccessToken(const std::string& t) { accessToken = t; }

include/mbgl/storage/resource.hpp

@@ -47,8 +47,8 @@ public:
     static Resource spriteImage(const std::string& base, float pixelRatio);
     static Resource spriteJSON(const std::string& base, float pixelRatio);
 
-    const Kind kind;
-    const std::string url;
+    Kind kind;
+    std::string url;
 
     // Includes auxiliary data if this is a tile request.
     optional<TileData> tileData;