Add support for loading pak files from arbitrary file regions.

base/files/file.h

Index: base/files/file.h
diff --git a/base/files/file.h b/base/files/file.h
index 1913bc7f6c408c8e1e3840da712895ff2fd88259..9bd70ed2d845cb001cbeb8d74639a2354f7dfc96 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -155,6 +155,26 @@ class BASE_EXPORT File {
     base::Time creation_time;
   };
 
+  // Used to hold information about a region [offset + size] of a file.
+  struct BASE_EXPORT Region {
+    static const Region kWholeFile;
+
+    Region(int64 offset, int64 size);
+
+    bool operator==(const Region& other) const;
+
+    // Start of the region (measured in bytes from the beginning of the file).
+    int64 offset;
+
+    // Length of the region in bytes.
+    int64 size;
+
+  private:
+    // This ctor is used only by kWholeFile, to construct a zero-sized Region
+    // (which is forbidden by the public ctor) and uniquely identify kWholeFile.
+    Region();
+  };
+
   File();
 
   // Creates or opens the given file. This will fail with 'access denied' if the