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..7bf2bcf659de3c38ab2174ae78aaa5e99e462b66 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -155,6 +155,22 @@ class BASE_EXPORT 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