Fix failing invalidation test condition for FlatTreeStyleRecalc.

Document node is not marked with ChildNeedsStyleRecalc when
FlatTreeStyleRecalc is enabled. Test StyleEngine::NeedsStyleRecalc
instead.

Bug: 972752
Change-Id: I9d3276c76d74d7e1c9f44bbc1a30dff318aebce2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841954Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703285}

third_party/blink/renderer/core/css/invalidation/pending_invalidations_test.cc

@@ -56,7 +56,7 @@ TEST_F(PendingInvalidationsTest, ScheduleOnDocumentNode) {
   EXPECT_FALSE(GetDocument().NeedsStyleInvalidation());
   EXPECT_FALSE(GetDocument().ChildNeedsStyleInvalidation());
   EXPECT_FALSE(GetDocument().NeedsStyleRecalc());
-  EXPECT_TRUE(GetDocument().ChildNeedsStyleRecalc());
+  EXPECT_TRUE(GetStyleEngine().NeedsStyleRecalc());
 
   GetDocument().View()->UpdateAllLifecyclePhases(
       DocumentLifecycle::LifecycleUpdateReason::kTest);