Add infrastructure for partial layouts

Source/core/rendering/SubtreeLayoutScope.cpp

Index: Source/core/rendering/SubtreeLayoutScope.cpp
diff --git a/Source/core/rendering/SubtreeLayoutScope.cpp b/Source/core/rendering/SubtreeLayoutScope.cpp
index 7a35c5daf865277774ca5beecb2f873a1c2cc088..716421fa79da37935cb3ba4fce314e52f31cad64 100644
--- a/Source/core/rendering/SubtreeLayoutScope.cpp
+++ b/Source/core/rendering/SubtreeLayoutScope.cpp
@@ -44,6 +44,12 @@ SubtreeLayoutScope::SubtreeLayoutScope(RenderObject* root)
 
 SubtreeLayoutScope::~SubtreeLayoutScope()
 {
+    // Partial layout early-exits layout and will leave the tree as needing layout.
+    if (m_root->frameView()->partialLayout().isStopping()) {
+        ASSERT(m_root->needsLayout());
+        return;
+    }
+
     RELEASE_ASSERT(!m_root->needsLayout());
 
 #ifndef NDEBUG