(WIP) Paint invalidation for slimming paint v2

Source/platform/graphics/paint/DisplayItemList.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "platform/graphics/paint/DisplayItemList.h"
 
 #include "platform/NotImplemented.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/TraceEvent.h"
@@ skipping 99 matching lines @@
 
 void DisplayItemList::endScope()
 {
     m_scopeStack.removeLast();
     endSkippingCache();
 }
 
 void DisplayItemList::invalidate(DisplayItemClient client)
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintEnabled());
-    // Can only be called during layout/paintInvalidation, not during painting.
-    ASSERT(m_newDisplayItems.isEmpty());
+    // Paint invalidation may occur before or during painting. If the latter,
+    // we should not invalidate any client which has already has display items.
+    ASSERT(!m_newDisplayItemIndicesByClient.contains(client));
     updateValidlyCachedClientsIfNeeded();
     m_validlyCachedClients.remove(client);
     updateSubtreeIndicesByClientIfNeeded();
     m_subtreeIndicesByClient.remove(client);
 }
 
 void DisplayItemList::invalidateAll()
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintEnabled());
     // Can only be called during layout/paintInvalidation, not during painting.
@@ skipping 441 matching lines @@
 
 void DisplayItemList::replay(GraphicsContext& context)
 {
     TRACE_EVENT0("blink,benchmark", "DisplayItemList::replay");
     ASSERT(m_newDisplayItems.isEmpty());
     for (DisplayItem& displayItem : m_currentDisplayItems)
         displayItem.replay(context);
 }
 
 } // namespace blink