Disallow calling Map on currently-mapped SharedMemory

base/memory/shared_memory.h

Index: base/memory/shared_memory.h
diff --git a/base/memory/shared_memory.h b/base/memory/shared_memory.h
index 8e32e7267334b0367dc0f97eb5a60f30e4382e44..27c3747f8f972ae3bfe16fe46ac2a84ea86a6b53 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -154,7 +154,8 @@ class BASE_EXPORT SharedMemory {
   // Maps the shared memory into the caller's address space.
   // Returns true on success, false otherwise.  The memory address
   // is accessed via the memory() accessor.  The mapped address is guaranteed to
-  // have an alignment of at least MAP_MINIMUM_ALIGNMENT.
+  // have an alignment of at least MAP_MINIMUM_ALIGNMENT. This method will fail
+  // if this object is currently mapped.
   bool Map(size_t bytes) {
     return MapAt(0, bytes);
   }