[WebRTC] Mark Perfect Negotiation WPTs as flaky.

This will likely stop flaking when
https://chromium-review.googlesource.com/c/chromium/src/+/2323359
lands. But let's wait for the follow-up CL
https://chromium-review.googlesource.com/c/chromium/src/+/2330014
to land when we pass Perfect Negotiation WPTs before we enable the test
again just to be sure...

TBR=hta@chromium.org

Bug: chromium:1060083
Change-Id: Ic52f2e06f637d005e0f1fd83fcfdab82d2731296
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332577Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793580}

third_party/blink/web_tests/TestExpectations

@@ -2657,6 +2657,9 @@ crbug.com/27659 external/wpt/css/css-ruby/ruby-line-breaking-003.html [ Failure
 crbug.com/27659 external/wpt/css/css-ruby/ruby-whitespace-001.html [ Failure ]
 crbug.com/27659 external/wpt/css/css-ruby/ruby-whitespace-002.html [ Failure ]
 
+# WebRTC: Perfect Negotiation is flaky in Unified Plan.
+# This will be fixed when crbug.com/1060083 is fixed.
+crbug.com/1060083 external/wpt/webrtc/RTCPeerConnection-perfect-negotiation.https.html [ Pass Failure Timeout ]
 # WebRTC: Perfect Negotiation times out in Plan B. This is expected.
 crbug.com/980872 virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCPeerConnection-perfect-negotiation.https.html [ Timeout ]