[Sheriff] Update TestExpectations

virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html
is timing out on Mac

Bug: 626703
Change-Id: I6344a749f9a7de6653cc266382faf8a9f1116acd
TBR: hta@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149325Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759209}

third_party/blink/web_tests/TestExpectations

@@ -3151,6 +3151,7 @@ crbug.com/626703 [ Mac10.14 ] external/wpt/webrtc/RTCRtpSender-replaceTrack.http
 crbug.com/626703 [ Win7 ] external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html [ Timeout ]
 crbug.com/626703 [ Win10 ] external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html [ Timeout ]
 crbug.com/626703 [ Win ] virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html [ Timeout ]
+crbug.com/626703 [ Mac ] virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html [ Timeout ]
 crbug.com/626703 [ Linux ] external/wpt/css/css-text/line-break/line-break-normal-015a.xht [ Failure ]
 crbug.com/626703 [ Mac ] external/wpt/css/css-text/line-break/line-break-normal-015a.xht [ Failure ]
 crbug.com/626703 [ Win ] external/wpt/css/css-text/line-break/line-break-normal-015a.xht [ Failure ]