[Sheriff] Disable failing webrtc perf tests

Disabling webrtc/multiple_peerconnections on linux and win10,
and disabling webrtc/pause_play_peerconnections on linux.

Notry: true
Bug: 1081349
Change-Id: I905824011d5c0a0c13c75ed13121021f8f5af6a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2196941
Auto-Submit: John Chen <johnchen@chromium.org>
Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Cr-Commit-Position: refs/heads/master@{#767854}

tools/perf/expectations.config

@@ -412,6 +412,9 @@ crbug.com/1064658 [ android ] v8.browsing_mobile-future/browse:tech:discourse_in
 crbug.com/954229 [ mac ] v8.runtime_stats.top_25/* [ Skip ]
 
 # Benchmark: webrtc
+crbug.com/1081349 [ linux ] webrtc/multiple_peerconnections [ Skip ]
+crbug.com/1081349 [ win10 ] webrtc/multiple_peerconnections [ Skip ]
+crbug.com/1081349 [ linux ] webrtc/pause_play_peerconnections [ Skip ]
 crbug.com/1051644 [ win ] webrtc/pause_play_peerconnections [ Skip ]
 
 ##### Perf FYI benchmarks go after here #####