Advertise a minimum alignment for SharedMemory::Map().

base/shared_memory.h

Index: base/shared_memory.h
diff --git a/base/shared_memory.h b/base/shared_memory.h
index e1c083377bb56f55d9cc6726d3148ba57b67afea..1f8237eb3001c6399e9cfe6faf0f8e9548986b3a 100644
--- a/base/shared_memory.h
+++ b/base/shared_memory.h
@@ -142,8 +142,10 @@ 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.
+  // is accessed via the memory() accessor.  The mapped address is guaranteed to
+  // have an alignment of at least MAP_MINIMUM_ALIGNMENT.
   bool Map(uint32 bytes);
+  enum { MAP_MINIMUM_ALIGNMENT = 32 };
 
   // Unmaps the shared memory from the caller's address space.
   // Returns true if successful; returns false on error or if the