Introduce runtime flag for flat tree order style recalc.

Bug: 972752
Change-Id: I61ae4496ac0438e3ebf192dd3f44550e6862748c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722949Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681759}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -639,6 +639,11 @@
       name: "FirstContentfulPaintPlusPlus",
       status: "stable",
     },
+    // Perform style recalc traversal in the flat tree order, including marking
+    // flat-tree instead of shadow-including ancestors for child dirtiness.
+    {
+      name: "FlatTreeStyleRecalc",
+    },
     {
       name: "FocuslessSpatialNavigation",
       settable_from_internals: true,