[Traffic Annotations] Whitelist udp_socket.cc for CreateMutable...

The new `python_script' backend for network annotation buildbots checks
calls to CreateMutableNetworkTrafficAnnotationTag(), except for
whitelisted files. This CL updates the whitelist for the upcoming
migration to the new backend.

TBR=rhalavati@chromium.org

Bug: 966883
Change-Id: Ib61fa214230a45b8d638f0626c3c2c9dfb5ab1c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760886
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688667}

tools/traffic_annotation/auditor/safe_list.txt

@@ -7,6 +7,7 @@ missing,net/tools/quic/quic_http_proxy_backend_stream.cc
 missing,net/url_request/url_fetcher.cc
 missing,net/url_request/url_request_context.cc
 mutable_tag,components/download/internal/background_service/proto_conversions.cc
+mutable_tag,chrome/browser/media/router/providers/openscreen/platform/udp_socket.cc
 test_annotation,components/safe_search_api/stub_url_checker.cc
 test_annotation,net/quic/quic_chromium_client_session_peer.cc
 test_annotation,net/tools/quic/quic_http_proxy_backend_stream.cc