Add support for loading pak files from arbitrary file regions.

base/files/memory_mapped_file.cc

Index: base/files/memory_mapped_file.cc
diff --git a/base/files/memory_mapped_file.cc b/base/files/memory_mapped_file.cc
index ace4e112628519137e7f56fbfb36f6176c2e8e83..01de9248138c081f73883b815a69638bb118a6f8 100644
--- a/base/files/memory_mapped_file.cc
+++ b/base/files/memory_mapped_file.cc
@@ -24,7 +24,7 @@ bool MemoryMappedFile::Initialize(const FilePath& file_name) {
     return false;
   }
 
-  if (!MapFileToMemory()) {
+  if (!MapFileToMemory(base::File::Region(file_))) {
     CloseHandles();
     return false;
   }
@@ -33,12 +33,17 @@ bool MemoryMappedFile::Initialize(const FilePath& file_name) {
 }
 
 bool MemoryMappedFile::Initialize(File file) {
+  const base::File::Region whole_file(file);
+  return Initialize(file.Pass(), whole_file);
+}
+
+bool MemoryMappedFile::Initialize(File file, const base::File::Region& region) {
   if (IsValid())
     return false;
 
   file_ = file.Pass();
 
-  if (!MapFileToMemory()) {
+  if (!MapFileToMemory(region)) {
     CloseHandles();
     return false;
   }