Restore platform/mediastream OWNERS file

File was removed (accidentally) in https://crrev.com/c/1745279,
instead of getting moved down to Blink.

R=jbroman@chromium.org

Change-Id: I09da94c97c1dc04613bdf6714d01afa01f798016
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1950624
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#721502}

third_party/blink/renderer/platform/mediastream/OWNERS

+file://third_party/blink/common/mediastream/OWNERS
+
+per-file media_stream_audio_processor*=aluebs@chromium.org
+
+# TEAM: webrtc-dev@chromium.org
+# COMPONENT: Blink>GetUserMedia