Add support for loading pak files from arbitrary file regions.

ui/base/resource/data_pack.cc

 // 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.
 
 #include "ui/base/resource/data_pack.h"
 
 #include <errno.h>
 
 #include "base/file_util.h"
 #include "base/files/memory_mapped_file.h"
@@ skipping 67 matching lines @@
     DLOG(ERROR) << "Failed to mmap datapack";
     UMA_HISTOGRAM_ENUMERATION("DataPack.Load", INIT_FAILED,
                               LOAD_ERRORS_COUNT);
     mmap_.reset();
     return false;
   }
   return LoadImpl();
 }
 
 bool DataPack::LoadFromFile(base::File file) {
+  const base::File::Region whole_file(file);
+  return LoadFromFileRegion(file.Pass(), whole_file);
+}
+
+bool DataPack::LoadFromFileRegion(base::File file,
+                                  const base::File::Region& region) {
   mmap_.reset(new base::MemoryMappedFile);
-  if (!mmap_->Initialize(file.Pass())) {
+  if (!mmap_->Initialize(file.Pass(), region)) {
     DLOG(ERROR) << "Failed to mmap datapack";
     UMA_HISTOGRAM_ENUMERATION("DataPack.Load", INIT_FAILED_FROM_FILE,
                               LOAD_ERRORS_COUNT);
     mmap_.reset();
     return false;
   }
   return LoadImpl();
 }
 
 bool DataPack::LoadImpl() {
@@ skipping 209 matching lines @@
       return false;
     }
   }
 
   base::CloseFile(file);
 
   return true;
 }
 
 }  // namespace ui