Add DIR_METADATA for WebCodecs project.

This allows the fuzzers to assign the right component as well as
other integrations.

R=tguilbert

Bug: None
Change-Id: I16e4b79d479ef1a704b31b3d1bf91e687a7c5ae0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638453
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844920}

third_party/blink/renderer/modules/webcodecs/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>Media>WebCodecs"
+}