Mark play-state-initially-paused-start-event.html as flaky.

This cl marks animations/events/play-state-initially-paused-start-event.html
as flaky.
sampe flakes:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=animations%2Fevents%2Fplay-state-initially-paused-start-event.html

NOTRY=TRUE

Bug: 994746
Change-Id: I6238f233ac6e4ff524b6472b7a6e9153cd02c5e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758214Reviewed-by: Sahel Sharify <sahel@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687677}

third_party/blink/web_tests/TestExpectations

@@ -6450,6 +6450,7 @@ crbug.com/993238 fast/frames/frame-name-reset.html [ Pass Failure ]
 
 #Sherrif 2019-08-16
 crbug.com/994692 [ Linux Win ] compositing/reflections/nested-reflection-anchor-point.html [ Pass Failure ]
+crbug.com/994746 animations/events/play-state-initially-paused-start-event.html [ Pass Timeout ]
 
 # Video flakily not being being rendered on Mac
 crbug.com/979298 [ Mac ] media/controls/captions-menu-always-visible.html [ Pass Failure ]