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

base/shared_memory_posix.cc

Index: base/shared_memory_posix.cc
diff --git a/base/shared_memory_posix.cc b/base/shared_memory_posix.cc
index c948e58f2d134231d69c09890348342ff4e0502a..37f0c4ec1933f4b59857b59f19b8df90b7400ed3 100644
--- a/base/shared_memory_posix.cc
+++ b/base/shared_memory_posix.cc
@@ -65,6 +65,11 @@ bool SharedMemory::IsHandleValid(const SharedMemoryHandle& handle) {
   return handle.fd >= 0;
 }
 
+// static
+SharedMemoryHandle SharedMemory::NULLHandle() {
+  return SharedMemoryHandle();
+}
+
 bool SharedMemory::Create(const std::wstring &name, bool read_only,
                           bool open_existing, size_t size) {
   read_only_ = read_only;