Add support for loading pak files from arbitrary file regions.

ui/base/resource/data_pack.cc

Index: ui/base/resource/data_pack.cc
diff --git a/ui/base/resource/data_pack.cc b/ui/base/resource/data_pack.cc
index 8e9ee30eef60b64714ba186750ddf50b301d3336..d0a153b1daa9dcdb3e2126a4fa370cb438c32c23 100644
--- a/ui/base/resource/data_pack.cc
+++ b/ui/base/resource/data_pack.cc
@@ -85,8 +85,13 @@ bool DataPack::LoadFromPath(const base::FilePath& path) {
 }
 
 bool DataPack::LoadFromFile(base::File file) {
+  return LoadFromFileRegion(file.Pass(), base::File::Region::kWholeFile);
+}
+
+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);