[IntersectionObserver] Enable feature to accept Document as root

Approved intent-to-ship:

https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/6dOOw2vu1is

BUG=1015183

Change-Id: I5231d4014e4ac260a78c3328df6a1b8171e5e598
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015676Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735715}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -857,7 +857,7 @@
     },
     {
       name: "IntersectionObserverDocumentScrollingElementRoot",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "IntersectionObserverV2",