[:is/:where] Enable for stable

I2S: https://groups.google.com/a/chromium.org/g/blink-dev/c/Wodp9dSSdL8/m/OZo2vJqTAwAJ
Fixed: 568705, 808904, 1127343, 1127347
Change-Id: Ib81bf8243cc57124ac5e781e5d6da5d4b09d7edf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484600Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818586}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -570,11 +570,11 @@
     },
     {
       name: "CSSPseudoIs",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "CSSPseudoWhere",
-      status: "experimental",
+      status: "stable",
     },
     {
       // Perform partial layout invalidation on web font loading.