Rebaseline: Mark web-animations-api/player-finish-sample-only.html as flaky

BUG=581203
TBR=dstockwell@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1632103002

Cr-Commit-Position: refs/heads/master@{#371360}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1675,3 +1675,5 @@ crbug.com/579151 editing/deleting/delete-to-select-table.html [ NeedsRebaseline
 crbug.com/579151 editing/inserting/insert-div-018.html [ NeedsRebaseline ]
 
 crbug.com/581038 fast/text/first-letter-bad-line-boxes-crash.html [ Crash Pass ]
+
+crbug.com/581203 [ Linux ] web-animations-api/player-finish-sample-only.html [ Failure Pass ]
\ No newline at end of file