Add FileSystemMountPointProvider::CreateFileReader() which returns appropriate Reader instance

webkit/fileapi/file_system_context.h

Index: webkit/fileapi/file_system_context.h
diff --git a/webkit/fileapi/file_system_context.h b/webkit/fileapi/file_system_context.h
index 35e043debe7f3b8999847632375c2e4c72bec2aa..d72a3ca745fdef0a1e2b6d2d962bbeca866482a3 100644
--- a/webkit/fileapi/file_system_context.h
+++ b/webkit/fileapi/file_system_context.h
@@ -25,6 +25,10 @@ namespace quota {
 class QuotaManagerProxy;
 }
 
+namespace webkit_blob {
+class FileReader;
+}
+
 namespace fileapi {
 
 class ExternalFileSystemMountPointProvider;
@@ -114,6 +118,17 @@ class FileSystemContext
       const GURL& url,
       base::MessageLoopProxy* file_proxy);
 
+  // Creates new FileReader instance to read a file pointed by the given
+  // filesystem URL |url| starting from |offset|.
+  // This method internally cracks the |url|, get an appropriate
+  // MountPointProvider for the URL and call the provider's CreateFileReader.
+  // The resolved MountPointProvider could perform further specialization
+  // depending on the filesystem type pointed by the |url|.
+  webkit_blob::FileReader* CreateFileReader(
+      const GURL& url,
+      int64 offset,
+      base::MessageLoopProxy* file_proxy);
+
  private:
   friend struct DefaultContextDeleter;
   void DeleteOnCorrectThread() const;