Fix -Wdefaulted-function-deleted warning in StreamPrioKey

../../third_party/webrtc/modules/pacing/round_robin_packet_queue.h:70:5:
warning: explicitly defaulted default constructor is implicitly deleted
[-Wdefaulted-function-deleted]
    StreamPrioKey() = default;
    ^
../../third_party/webrtc/modules/pacing/round_robin_packet_queue.h:80:37: note:
default constructor of 'StreamPrioKey' is implicitly deleted because field
'priority' of const-qualified type 'const RtpPacketSender::Priority' would not
be initialized
    const RtpPacketSender::Priority priority;
                                    ^

Bug: chromium:890307
Change-Id: I58f21121fc9083a60ba1ad26492fdca6285d0447
Reviewed-on: https://webrtc-review.googlesource.com/c/103181
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24952}