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..4fecd8f873e0b651520a1cc8c4c94f65490b4ff2 100644
--- a/base/files/memory_mapped_file.cc
+++ b/base/files/memory_mapped_file.cc
@@ -6,6 +6,7 @@
 
 #include "base/files/file_path.h"
 #include "base/logging.h"
+#include "base/sys_info.h"
 
 namespace base {
 
@@ -24,7 +25,7 @@ bool MemoryMappedFile::Initialize(const FilePath& file_name) {
     return false;
   }
 
-  if (!MapFileToMemory()) {
+  if (!MapFileRegionToMemory(File::Region::WholeFile())) {
     CloseHandles();
     return false;
   }
@@ -33,12 +34,16 @@ bool MemoryMappedFile::Initialize(const FilePath& file_name) {
 }
 
 bool MemoryMappedFile::Initialize(File file) {
+  return Initialize(file.Pass(), base::File::Region::WholeFile());
+}
+
+bool MemoryMappedFile::Initialize(File file, const base::File::Region& region) {
   if (IsValid())
     return false;
 
   file_ = file.Pass();
 
-  if (!MapFileToMemory()) {
+  if (!MapFileRegionToMemory(region)) {
     CloseHandles();
     return false;
   }
@@ -50,4 +55,17 @@ bool MemoryMappedFile::IsValid() const {
   return data_ != NULL;
 }
 
+// static
+void MemoryMappedFile::CalculateVMAlignedBoundaries(int64 start,
+                                                    int64 size,
+                                                    int64* aligned_start,
+                                                    int64* aligned_size,
+                                                    int32* offset) {
+  // Sadly, on Windows, the mmap alignment is not just equal to the page size.
+  const int64 mask = static_cast<int64>(SysInfo::VMAllocationGranularity()) - 1;
+  *offset = start & mask;
+  *aligned_start = start & ~mask;
+  *aligned_size = (size + *offset + mask) & ~mask;
+}
+
 }  // namespace base