Add support for loading pak files from arbitrary file regions.

base/files/file.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_FILES_FILE_H_
 #define BASE_FILES_FILE_H_
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 137 matching lines @@
     // The last modified time of a file.
     base::Time last_modified;
 
     // The last accessed time of a file.
     base::Time last_accessed;
 
     // The creation time of a file.
     base::Time creation_time;
   };
 
+  // Used to hold information about a region [offset + size] of a file.
+  struct BASE_EXPORT Region {
+    Region(int64 offset, int64 size);
+
+    // Retrieves a region which covers the entire file.
+    explicit Region(File& file);

[Primiano Tucci (use gerrit), 2014/07/16 23:14:39]

Shame that this cannot be const. Unfortunately GetLength is deliberately non
const.
The alternative would be passing as const here and reinterpret_cast in the ctor,
which sounds a bit odd. :-/
Or eventually just a naked pointer. I feel the non-cost reference is the
cleanest option.

+
+    ~Region();
+
+    // Start of the region (measured in bytes from the beginning of the file).
+    int64 offset;
+
+    // Length of the region.
+    int64 size;
+  };
+
   File();
 
   // Creates or opens the given file. This will fail with 'access denied' if the
   // |name| contains path traversal ('..') components.
   File(const FilePath& name, uint32 flags);
 
   // Takes ownership of |platform_file|.
   explicit File(PlatformFile platform_file);
 
   // Creates an object with a specific error_details code.
@@ skipping 136 matching lines @@
 #endif
 
   Error error_details_;
   bool created_;
   bool async_;
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_