Mark test as flaky until it can be fixed.

external/wpt/pointerevents/pointerevent_capture_mouse.html
is failing in blink_web_tests on WebKit Linux Leak

Bug: 998948
Change-Id: I32248d6cf9183605a4fd5b5ebaf959af6179a75c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2200007
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768533}

third_party/blink/web_tests/TestExpectations

@@ -5934,3 +5934,7 @@ crbug.com/984467 [ Mac Debug ] virtual/gpu/fast/canvas/canvas-composite-stroke-a
 # Sheriff 2020-05-06
 crbug.com/1078863 [ Mac ] fast/speech/scripted/speechrecognition-restart-onend.html [ Pass Timeout ]
 crbug.com/1071915 fast/hidpi/image-srcset-png-canvas.html [ Pass Failure ]
+
+# Sheriff 2020-05-13
+# Flaking on Webkit Linux Leak
+crbug.com/998948 [ Linux ] external/wpt/pointerevents/pointerevent_capture_mouse.html [ Pass Failure ]