[base] Drop MFD_CLOEXEC in OS::CreateSharedMemoryHandleForTesting

The flag is causing build failures in some environments (when it is not
defined) and it is not actually needed since the behaviour of these FDs
across fork/exec isn't defined and this code is only used for testing.

Bug: chromium:1218005
Change-Id: If2bf6835705318079af56d920017accedaf769dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3695583
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81032}

src/base/platform/platform-posix.cc

@@ -610,8 +610,9 @@ PlatformSharedMemoryHandle OS::CreateSharedMemoryHandleForTesting(size_t size) {
       reinterpret_cast<memfd_create_t>(dlsym(RTLD_DEFAULT, "memfd_create"));
   int fd = -1;
   if (memfd_create) {
-    fd = memfd_create("V8MemFDForTesting", MFD_CLOEXEC);
-  } else {
+    fd = memfd_create("V8MemFDForTesting", 0);
+  }
+  if (fd == -1) {
     char filename[] = "/tmp/v8_tmp_file_for_testing_XXXXXX";
     fd = mkstemp(filename);
     if (fd != -1) CHECK_EQ(0, unlink(filename));