[meta] Add DIR_METADATA for src/maglev

Bug: v8:7700
Change-Id: I4efa8f8b3b7df03b3fb6b6bd35c7310b0da07d49
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3879613
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83067}

src/maglev/DIR_METADATA

+# Metadata information for this directory.
+#
+# For more information on DIR_METADATA files, see:
+#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/README.md
+#
+# For the schema of this file, see Metadata message:
+#   https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/proto/dir_metadata.proto
+
+monorail {
+  component: "Blink>JavaScript>Compiler>Maglev"
+}