Disallow calling Map on currently-mapped SharedMemory

base/memory/shared_memory_posix.cc

Index: base/memory/shared_memory_posix.cc
diff --git a/base/memory/shared_memory_posix.cc b/base/memory/shared_memory_posix.cc
index 1a90847145b4d688bbd9cfd6ceac510c3da5c4fd..1756c292adb28709d7a2c2f70e6fcf000ce80ddd 100644
--- a/base/memory/shared_memory_posix.cc
+++ b/base/memory/shared_memory_posix.cc
@@ -279,6 +279,9 @@ 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;
+
 #if defined(OS_ANDROID)
   // On Android, Map can be called with a size and offset of zero to use the
   // ashmem-determined size.