Revert "Enable implicit root scroller in stable"

This reverts commit 0d1087c7.

Reason for revert: Feature isn't approved to ship - was turned on to get some early data in Canary/Dev

Original change's description:
> Enable implicit root scroller in stable
> 
> Bug: 798719
> Change-Id: I05c0bc89c3e842f005be57db2226b4188ee4da3b
> Reviewed-on: https://chromium-review.googlesource.com/c/1320812
> Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
> Commit-Queue: David Bokan <bokan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#606428}

TBR=bokan@chromium.org,chrishtr@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 798719
Change-Id: I9fb19298e818fc21bfd42404253a5110a226246e
Reviewed-on: https://chromium-review.googlesource.com/c/1351604Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611133}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -619,7 +619,7 @@
     },
     {
       name: "ImplicitRootScroller",
-      status: "stable",
+      status: "experimental",
       settable_from_internals: true,
     },
     {