Turn off CompositeRelativeKeyframes in stable.

The CompositeRelativeKeyframes feature was turned on in stable in the
following CL.

https://chromium-review.googlesource.com/c/chromium/src/+/2495604

This change introduced a regression (crbug.com/1149012) in which the
animation onfinished event is not being fired.

Disabling the feature while we determine why the event is not firing.

Bug: 1149012, 389359

Change-Id: Ibe789132f0782fa12b129045777c53bd46596697
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2540568Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827858}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -357,7 +357,7 @@
     },
     {
       name: "CompositeRelativeKeyframes",
-      status: "stable",
+      status: "experimental",
     },
     {
       name: "CompositeSVG",