Enable CompositeRelativeKeyframes

Underlying problem that lead to the earlier revert fixed in:

https://chromium-review.googlesource.com/c/chromium/src/+/2614365




Bug: 389359
Change-Id: I0c688430e8a1df0f2af6711d58e883f2cd179fcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625290Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843080}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -356,7 +356,7 @@
     },
     {
       name: "CompositeRelativeKeyframes",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "CompositeSVG",