Mark RTCRtpSender-replaceTrack flaky on Mac 10.13

On that system (only), there is occasionally "Content-type: text/plain"
printed ahead of the test output.

Bug: chromium:1069714
Change-Id: Ibd7242c7f7064135de9ede87b7ca366899893599
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146539Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758975}

third_party/blink/web_tests/TestExpectations

@@ -3157,7 +3157,6 @@ crbug.com/626703 [ Win ] external/wpt/webxr/hit-test/ar_hittest_subscription_ref
 crbug.com/626703 [ Linux ] external/wpt/webxr/hit-test/ar_hittest_subscription_transientInputSources.https.html [ Timeout ]
 crbug.com/626703 [ Mac ] external/wpt/webxr/hit-test/ar_hittest_subscription_transientInputSources.https.html [ Timeout ]
 crbug.com/626703 [ Win ] external/wpt/webxr/hit-test/ar_hittest_subscription_transientInputSources.https.html [ Timeout ]
-crbug.com/626703 [ Mac10.13 ] external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html [ Timeout ]
 crbug.com/626703 [ Linux ] external/wpt/webxr/hit-test/ar_hittest_subscription_inputSources.https.html [ Timeout ]
 crbug.com/626703 [ Mac ] external/wpt/webxr/hit-test/ar_hittest_subscription_inputSources.https.html [ Timeout ]
 crbug.com/626703 [ Win ] external/wpt/webxr/hit-test/ar_hittest_subscription_inputSources.https.html [ Timeout ]
@@ -6670,5 +6669,9 @@ crbug.com/1067533 [ Mac10.10 ] external/wpt/shape-detection/idlharness.https.any
 crbug.com/1064422 external/wpt/html/user-activation/propagation-crossorigin.sub.tentative.html [ Pass Timeout ]
 crbug.com/1066178 external/wpt/html/user-activation/consumption-crossorigin.sub.tentative.html [ Pass Timeout ]
 
+# Flaky test, happens only on Mac 10.13.
+# The "timeout" was detected by the wpt-importer bot.
+crbug.com/1069714 [ Mac10.13 ] external/wpt/webrtc/RTCRtpSender-replaceTrack.https.html [ Pass Failure Timeout ]
+
 # Sheriff 2020-04-10
 crbug.com/1068681 fast/peerconnection/RTCPeerConnection-insertable-streams.html [ Pass Failure Timeout ]