Mark external/wpt/xhr/timeout-multiple-fetches.html as flaky

No-Try: True
Bug: 871578
Change-Id: I90e8d63d4d0424cfa189db3ba726022c7b30c6eb
Reviewed-on: https://chromium-review.googlesource.com/1164809Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581160}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4849,5 +4849,9 @@ crbug.com/871139 [ Android ] virtual/threaded/synthetic_gestures/synthetic-pinch
 crbug.com/871139 [ Android ] virtual/threaded/synthetic_gestures/synthetic-pinch-zoom-gesture-touchscreen.html [ Failure ]
 crbug.com/869726 [ Mac ] virtual/mouseevent_fractional/fast/events/touch/touch-latched-scroll-node-removed.html [ Pass Failure ]
 
+crbug.com/871578 [ Mac ] external/wpt/xhr/timeout-multiple-fetches.html [ Failure Pass ]
+crbug.com/871578 [ Mac ] virtual/outofblink-cors/external/wpt/xhr/timeout-multiple-fetches.html [ Failure Pass ]
+crbug.com/871578 [ Mac ] virtual/outofblink-cors-ns/external/wpt/xhr/timeout-multiple-fetches.html [ Failure Pass ]
+
 # Only passes on bots with GPUs.
 crbug.com/871445 [ Mac ] http/tests/media/video-load-metadata-decode-error.html [ Timeout ]