(WIP) Paint invalidation for slimming paint v2

Source/platform/graphics/paint/DisplayItemList.cpp

Index: Source/platform/graphics/paint/DisplayItemList.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemList.cpp b/Source/platform/graphics/paint/DisplayItemList.cpp
index 03b0ce50bbf8943f60766cdc54d5e63c4073bfba..9bc59d9234b9bf8eea9ed9d8629fbe39091859df 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -117,8 +117,9 @@ void DisplayItemList::endScope()
 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();