Port visitedlink tests to Linux. Also make them pass on Linux, which

base/shared_memory.h

Index: base/shared_memory.h
diff --git a/base/shared_memory.h b/base/shared_memory.h
index 744b3489e7d9d7fb21c6d7d5878019faa7091a86..bd6770c95698dd42d7f082b04b9d2ff5d20421eb 100644
--- a/base/shared_memory.h
+++ b/base/shared_memory.h
@@ -58,6 +58,9 @@ class SharedMemory {
   // invalid value; NULL for a HANDLE and -1 for a file descriptor)
   static bool IsHandleValid(const SharedMemoryHandle& handle);
 
+  // Return invalid handle (see comment above for exact definition).
+  static SharedMemoryHandle NULLHandle();
+
   // Creates or opens a shared memory segment based on a name.
   // If read_only is true, opens the memory as read-only.
   // If open_existing is true, and the shared memory already exists,