Enable implicit root scroller in stable

Bug: 798719
Change-Id: I05c0bc89c3e842f005be57db2226b4188ee4da3b
Reviewed-on: https://chromium-review.googlesource.com/c/1320812Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606428}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -597,7 +597,7 @@
     },
     {
       name: "ImplicitRootScroller",
-      status: "experimental",
+      status: "stable",
       settable_from_internals: true,
     },
     {