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..636aa88c3ed4f22ab7b96f636abba4b6961d07ef 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 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.
@@ -48,9 +51,21 @@ class BASE_EXPORT MemoryMappedFile {
   bool IsValid() const;
 
  private:
+  // Given the arbitrarily aligned memory region [start, size], returns the
+  // boundaries of the region aligned to the granularity specified by the OS,
+  // (a page on Linux, ~32k on Windows) as follows:
+  // - |aligned_start| is page aligned and <= |start|.
+  // - |aligned_size| is a multiple of the VM granularity and >= |size|.
+  // - |offset| is the displacement of |start| w.r.t |aligned_start|.
+  static void CalculateVMAlignedBoundaries(int64 start,
+                                           int64 size,
+                                           int64* aligned_start,
+                                           int64* aligned_size,
+                                           int32* offset);
+
   // 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 MapFileRegionToMemory(const File::Region& region);
 
   // Closes all open handles.
   void CloseHandles();