Add infrastructure for partial layouts

Source/core/rendering/SubtreeLayoutScope.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 26 matching lines @@
 namespace WebCore {
 
 SubtreeLayoutScope::SubtreeLayoutScope(RenderObject* root)
     : m_root(root)
 {
     RELEASE_ASSERT(m_root->document().view()->isInLayout());
 }
 
 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
     for (HashSet<RenderObject*>::iterator it = m_renderersToLayout.begin(); it != m_renderersToLayout.end(); ++it) {
         RenderObject* renderer = *it;
         // FIXME: Thie patter is really common. Move it into an assertRendererLaidOut function.
         if (renderer->needsLayout())
             showRenderTree(renderer);
         ASSERT(!renderer->needsLayout());
     }
@@ skipping 13 matching lines @@
 }
 
 void SubtreeLayoutScope::addRendererToLayout(RenderObject* renderer)
 {
 #ifndef NDEBUG
     m_renderersToLayout.add(renderer);
 #endif
 }
 
 }