Mark virtual/*/animations/play-state.html as flaky on Linux

This test is flaky due to crrev.com/c/1704389. Currently there is only
one occurance on Win7 so just mark it flaky on Linux for now.

See https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVySQsSBUZsYWtlIj5jaHJvbWl1bUBibGlua193ZWJfdGVzdHNfZXhwYXJjaGl2ZUBhbmltYXRpb25zL3BsYXktc3RhdGUuaHRtbAw

TBR=flackr@chromium.org
NOTRY=true

Bug: 986019
Change-Id: I637b1f43393cb9d1edd04684b9ac400b16fe348a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712055Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679552}

third_party/blink/web_tests/TestExpectations

@@ -6310,3 +6310,5 @@ crbug.com/986018 fast/scroll-snap/snaps-after-keyboard-scrolling-rtl.html [ Pass
 # Sheriff 2019-07-22
 crbug.com/986019 [ Linux Win ] virtual/threaded/animations/web-animations/animation-state-changes-positive-playback-rate.html [ Pass Failure ]
 crbug.com/986282 [ Linux Win ] external/wpt/client-hints/accept-ch-lifetime.tentative.https.html [ Pass Failure ]
+crbug.com/986019 [ Linux ] virtual/threaded/animations/play-state.html [ Pass Timeout ]
+crbug.com/986019 [ Linux ] virtual/disable-blink-gen-property-trees/animations/play-state.html [ Pass Timeout ]