Add support for loading pak files from arbitrary file regions.

base/files/memory_mapped_file.h

Index: base/files/memory_mapped_file.h
diff --git a/base/files/memory_mapped_file.h b/base/files/memory_mapped_file.h
index b02d8cfbdae09b89159675495313eae521a7576e..c6f80cceafdec3bf411e2a1e721adc74e35a25d9 100644
--- a/base/files/memory_mapped_file.h
+++ b/base/files/memory_mapped_file.h
@@ -35,6 +35,9 @@ class BASE_EXPORT MemoryMappedFile {
   // ownership of |file| and closes it when done.
   bool Initialize(File file);
 
+  // As above, but works with a region of an already-opened file.
+  bool Initialize(File file, const base::File::Region& region);
+
 #if defined(OS_WIN)
   // Opens an existing file and maps it as an image section. Please refer to
   // the Initialize function above for additional information.
@@ -50,7 +53,7 @@ class BASE_EXPORT MemoryMappedFile {
  private:
   // Map the file to memory, set data_ to that memory address. Return true on
   // success, false on any kind of failure. This is a helper for Initialize().
-  bool MapFileToMemory();
+  bool MapFileToMemory(const base::File::Region& region);
 
   // Closes all open handles.
   void CloseHandles();