Added SharedMemory::MapFrom.

base/shared_memory_win.cc

Index: base/shared_memory_win.cc
diff --git a/base/shared_memory_win.cc b/base/shared_memory_win.cc
index 877ccd76e3310370b0394f196cbb6bee9290c581..ea7f91e19ac67e75530338fe98a67eaadba65c8b 100644
--- a/base/shared_memory_win.cc
+++ b/base/shared_memory_win.cc
@@ -5,6 +5,7 @@
 #include "base/shared_memory.h"
 
 #include "base/logging.h"
+#include "base/sys_info.h"
 #include "base/utf_string_conversions.h"
 
 namespace base {
@@ -134,15 +135,19 @@ bool SharedMemory::Open(const std::string& name, bool read_only) {
   return false;
 }
 
-bool SharedMemory::Map(size_t bytes) {
+bool SharedMemory::MapAt(off_t offset, size_t bytes) {
   if (mapped_file_ == NULL)
     return false;
 
   if (bytes > static_cast<size_t>(std::numeric_limits<int>::max()))
     return false;
 
+  DCHECK_EQ(0U, offset & (SysInfo::VMAllocationGranularity() - 1));
   memory_ = MapViewOfFile(mapped_file_,
-      read_only_ ? FILE_MAP_READ : FILE_MAP_ALL_ACCESS, 0, 0, bytes);
+                          read_only_ ? FILE_MAP_READ : FILE_MAP_ALL_ACCESS,
+                          static_cast<uint64>(offset) >> 32,
+                          static_cast<DWORD>(offset),
+                          bytes);
   if (memory_ != NULL) {
     DCHECK_EQ(0U, reinterpret_cast<uintptr_t>(memory_) &
         (SharedMemory::MAP_MINIMUM_ALIGNMENT - 1));