sheriff: Disable flaky WPT RTCPeerConnection test.

external/wpt/webrtc/RTCPeerConnection-perfect-negotiation.https.html

TBR=hbos

Bug: 1123558
Change-Id: I3f570e0d6b6271d7d2777f8c2a220b2316668d2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387035Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803322}

third_party/blink/web_tests/TestExpectations

@@ -6645,3 +6645,6 @@ crbug.com/1122106 external/wpt/webrtc/protocol/crypto-suite.https.html [ Pass Ti
 crbug.com/1122742 http/tests/devtools/sources/debugger/source-frame-inline-breakpoint-decorations.js [ Pass Timeout ]
 
 crbug.com/1122582 external/wpt/html/cross-origin-opener-policy/coop-csp-sandbox-navigate.https.html [ Pass Failure ]
+
+# Sheriff 2020-09-01
+crbug.com/1123558 external/wpt/webrtc/RTCPeerConnection-perfect-negotiation.https.html [ Pass Timeout ]