Add assert that updateLayoutTree clears dirty bits.

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 0c62d74ae835d3206f7712a5c853869eb9d488dd..051d4d2969e4b60cc15fdbab7cc9e8f0d5ce3a9c 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -1683,6 +1683,30 @@ void Document::inheritHtmlAndBodyElementStyles(StyleRecalcChange change)
     }
 }
 
+#if ENABLE(ASSERT)
+static void assertLayoutTreeUpdated(Node& root)
+{
+    for (Node& node : NodeTraversal::inclusiveDescendantsOf(root)) {
+        // We leave some nodes with dirty bits in the tree because they don't
+        // matter like Comment and ProcessingInstruction nodes.
+        if (!node.isElementNode()
+            && !node.isTextNode()
+            && !node.isShadowRoot()
+            && !node.isDocumentNode())
+            continue;
+
+        ASSERT(!node.needsStyleRecalc());
+        ASSERT(!node.childNeedsStyleRecalc());
+        ASSERT(!node.childNeedsDistributionRecalc());
+        ASSERT(!node.needsStyleInvalidation());
+        ASSERT(!node.childNeedsStyleInvalidation());
+
+        for (ShadowRoot* shadowRoot = node.youngestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->olderShadowRoot())
+            assertLayoutTreeUpdated(*shadowRoot);
+    }
+}
+#endif
+
 void Document::updateLayoutTree(StyleRecalcChange change)
 {
     ASSERT(isMainThread());
@@ -1748,6 +1772,10 @@ void Document::updateLayoutTree(StyleRecalcChange change)
 
     ASSERT(!m_timeline->hasOutdatedAnimation());
 
+#if ENABLE(ASSERT)
+    assertLayoutTreeUpdated(*this);
+#endif
+
     TRACE_EVENT_END1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "RecalculateStyles", "elementCount", m_styleRecalcElementCounter);
     TRACE_EVENT_END1("blink", "Document::updateLayoutTree", "elementCount", m_styleRecalcElementCounter);
     InspectorInstrumentation::didRecalculateStyle(cookie, m_styleRecalcElementCounter);