Enable FlatTreeStyleRecalc.

Enables marking of flat tree, instead of light tree, ancestors child-
dirty for style recalc. This means we can skip traversing sub trees
for shadow host children which are not part of the flat tree.

Bug: 972752, 971588

Change-Id: I302e840aed435c9cf146d5e49a01388b9e8718a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715298
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714034}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -711,6 +711,7 @@
     // flat-tree instead of shadow-including ancestors for child dirtiness.
     {
       name: "FlatTreeStyleRecalc",
+      status: "stable",
     },
     {
       name: "FocuslessSpatialNavigation",