Label sub-sample-buffer-stitching.html as failing

The test was added in crrev.com/c/1446869 but has been flaky on Mac.
Also add "Pass" as an acceptable outcome for
RTCPeerConnection-track-stats.https.html.

TBR=rtoy@chromium.org

Bug: 929435
Change-Id: I37ef496d7d6f382ffad5dc8d75935d9faba8e225
Reviewed-on: https://chromium-review.googlesource.com/c/1457222
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Auto-Submit: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629799}

third_party/blink/web_tests/TestExpectations

@@ -6039,4 +6039,5 @@ crbug.com/927769 fast/webgl/OffscreenCanvas-webgl-preserveDrawingBuffer.html [ S
 
 # Sheriff 2019-02-06
 crbug.com/929122 [ Linux ] external/wpt/html/dom/interfaces.worker.html [ Timeout Failure ]
-crbug.com/929355 [ Linux ] external/wpt/webrtc/RTCPeerConnection-track-stats.https.html [ Failure ]
+crbug.com/929355 [ Linux ] external/wpt/webrtc/RTCPeerConnection-track-stats.https.html [ Pass Failure ]
+crbug.com/929435 [ Mac ] external/wpt/webaudio/the-audio-api/the-audiobuffersourcenode-interface/sub-sample-buffer-stitching.html [ Pass Failure ]