Add DIR_METADATA to top directory.

Set project to "chromium" so it doesn't need to be specified for all
directories.

Bug: 1102997
Change-Id: I487ed21929db86c09d7b385343bee7a7243dd9b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2320890Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792450}

DIR_METADATA

+# Metadata for this directory.
+#
+# DIR_METADATA files allow code owners to point users to additional bits of
+# information about the code, like where to file bugs and how to contact the
+# team that works on the code.
+#
+# 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 {
+  project: "chromium"
+}