[Sheriff] Mark fast/peerconnection/RTCPeerConnection-iframe-gc.html as flaky

Please see linked bug for details.

TBR=hta@chromium.org
NOTRY=TRUE

Bug: 1091885
Change-Id: I13b7ee2dfbeef2f86a33748f64140f6a9d13712d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2232755Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775700}

third_party/blink/web_tests/TestExpectations

@@ -5935,3 +5935,4 @@ crbug.com/1057060 fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.htm
 # Sheriff 2020-06-05
 crbug.com/1091829 [ Mac ] fast/scrolling/scroll-non-composited-scroller.html [ Pass Failure ]
 crbug.com/1091829 [ Linux ] virtual/threaded-prefer-compositing/fast/scrolling/scroll-non-composited-scroller.html [ Pass Failure ]
+crbug.com/1091885 fast/peerconnection/RTCPeerConnection-iframe-gc.html [ Pass Failure ]
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