FORK: Disable WebAnimationsCSS

Work around internal issue 12393371 while investigation continues.

Revert of upstream https://codereview.chromium.org/96243003

Bug: 12393371
Change-Id: Ia0ef6fc84d12f257eff4cdd55cb50e9abc00f941
diff --git a/Source/platform/RuntimeEnabledFeatures.in b/Source/platform/RuntimeEnabledFeatures.in
index 91674bb..72a2261 100644
--- a/Source/platform/RuntimeEnabledFeatures.in
+++ b/Source/platform/RuntimeEnabledFeatures.in
@@ -123,7 +123,7 @@
 UserSelectAll status=experimental
 Vibration status=stable
 VideoTrack depends_on=Media, status=stable
-WebAnimationsCSS status=stable
+WebAnimationsCSS status=experimental
 WebAnimationsSVG depends_on=WebAnimationsCSS
 WebAnimationsAPI depends_on=WebAnimationsCSS, status=experimental
 WebAudio condition=WEB_AUDIO, status=stable