Enable CSSCacade on stable

Let's turn this on for a while to see if any UMA numbers move in Canary.
If they don't, we can leave it on. If they do, we still have a few weeks
until the next branch point.

Change-Id: I9e1432bb31ed1b6960e636df03a061dfd87735c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066786Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743511}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -411,7 +411,7 @@
     },
     {
       name: "CSSCascade",
-      status: "experimental",
+      status: "stable",
     },
     {
       // Support for the CSS color-scheme property from the css-color-adjust spec.