Mark opacity-responsive-neutral-keyframe.html as no longer flaky

This test is no longer flaky after being deflaked in
https://codereview.chromium.org/2346163003.

Flakiness dashboard:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fthreaded%2Fanimations%2Fopacity-responsive-neutral-keyframe.html

BUG=647776
TBR=ielelland@chromium.org

Review-Url: https://codereview.chromium.org/2350303006
Cr-Commit-Position: refs/heads/master@{#420000}

third_party/WebKit/LayoutTests/TestExpectations

@@ -238,7 +238,6 @@ crbug.com/248938 [ Mac ] virtual/threaded/transitions/transition-end-event-unpre
 crbug.com/513143 virtual/threaded/fast/scroll-behavior/subframe-interrupted-scroll.html [ Failure Pass ]
 crbug.com/248938 virtual/threaded/animations/display-none-terminates-animation.html [ Pass Failure ]
 crbug.com/638693 virtual/threaded/animations/display-inline-style-adjust.html [ Pass Crash Failure ]
-crbug.com/647776 [ Win7 Debug ] virtual/threaded/animations/opacity-responsive-neutral-keyframe.html [ Pass Failure ]
 crbug.com/421283 fast/html/marquee-scrollamount.html [ Pass Failure ]
 crbug.com/248938 [ Win Debug ] virtual/threaded/animations/delay-start-event.html [ Pass Failure ]
 crbug.com/248938 [ Mac Win ] virtual/threaded/transitions/transition-end-event-transform.html [ Pass Failure ]