Mark external/wpt/webrtc-quic/RTCQuicStream.https.html as Slow.

Bug: 925600
Change-Id: I2a5f9fbc8695f9205d7cdaf58b5eca3e4f168ab0
Tbr: msramek@chromium.org
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1438795Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626455}

third_party/blink/web_tests/SlowTests

@@ -1579,3 +1579,5 @@ crbug.com/914981 [ Mac ] virtual/mouseevent_fractional/fast/events/no-fake-mouse
 crbug.com/914981 [ Mac ] virtual/new-remote-playback-pipeline/media/controls/overflow-menu-pointer-selection.html [ Slow ]
 crbug.com/914981 [ Mac ] virtual/video-surface-layer/media/controls/overflow-menu-pointer-selection.html [ Slow ]
 crbug.com/914981 [ Mac ] fast/events/no-fake-mousemove.html [ Slow ]
+
+crbug.com/925600 external/wpt/webrtc-quic/RTCQuicStream.https.html [ Slow ]