Add support for loading pak files from arbitrary file regions.

ui/base/resource/data_pack.h

Index: ui/base/resource/data_pack.h
diff --git a/ui/base/resource/data_pack.h b/ui/base/resource/data_pack.h
index 4d9d4bbdc2f9eea9012b20a0a25af8de09626a0d..f9c4ef490f4b8e6b62b48ea35e861860317f09b3 100644
--- a/ui/base/resource/data_pack.h
+++ b/ui/base/resource/data_pack.h
@@ -38,6 +38,9 @@ class UI_BASE_EXPORT DataPack : public ResourceHandle {
   // Loads a pack file from |file|, returning false on error.
   bool LoadFromFile(base::File file);
 
+  // Loads a pack file from |region| of |file|, returning false on error.
+  bool LoadFromFileRegion(base::File file, const base::File::Region& region);
+
   // Writes a pack file containing |resources| to |path|. If there are any
   // text resources to be written, their encoding must already agree to the
   // |textEncodingType| specified. If no text resources are present, please