Disallow calling Map on currently-mapped SharedMemory

base/memory/shared_memory_nacl.cc

Index: base/memory/shared_memory_nacl.cc
diff --git a/base/memory/shared_memory_nacl.cc b/base/memory/shared_memory_nacl.cc
index 764740036c98844114a61e6a919c74810937c2fe..39625ee65b5087633a6727e9e7515192be227bc3 100644
--- a/base/memory/shared_memory_nacl.cc
+++ b/base/memory/shared_memory_nacl.cc
@@ -91,6 +91,9 @@ 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_ = mmap(NULL, bytes, PROT_READ | (read_only_ ? 0 : PROT_WRITE),
                  MAP_SHARED, mapped_file_, offset);