Disallow calling Map on currently-mapped SharedMemory

base/memory/shared_memory_win.cc

Index: base/memory/shared_memory_win.cc
diff --git a/base/memory/shared_memory_win.cc b/base/memory/shared_memory_win.cc
index c9d603ef8692146d2a001d62d39fbe23d03e0c95..cc177ab3f2155625965c117b00947282d566af6d 100644
--- a/base/memory/shared_memory_win.cc
+++ b/base/memory/shared_memory_win.cc
@@ -164,6 +164,9 @@ bool SharedMemory::MapAt(off_t offset, size_t bytes) {
   if (bytes > static_cast<size_t>(std::numeric_limits<int>::max()))
     return false;
 
+  if (memory_)
+    return false;
+
   memory_ = MapViewOfFile(mapped_file_,
                           read_only_ ? FILE_MAP_READ : FILE_MAP_READ |
                               FILE_MAP_WRITE,