Add a release assert that we found a cached item.

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 242 matching lines @@
             && !currentIt->isGone()
             && currentIt->id().equalToExceptForType(newDisplayItem.id(), matchingType);
 
         if (DisplayItem::isCachedType(newDisplayItem.type())) {
             ASSERT(!RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled());
             ASSERT(clientCacheIsValid(newDisplayItem.client()));
             if (isSynchronized) {
                 updatedList.appendByMoving(currentIt);
             } else {
                 DisplayItems::Iterator foundIt = findOutOfOrderCachedItem(currentIt, newDisplayItem.id(), matchingType, displayItemIndicesByClient);
-                ASSERT(foundIt != currentEnd);
                 isSynchronized = (foundIt == currentIt);
+
+                // TODO(chrishtr): downgrade to ASSERT as this goes to beta users, and replace with bailing out of the rest of the merge.
+                // This assert hits if we couldn't find the cached display item in our cache, which should be impossible.
+                RELEASE_ASSERT(foundIt != currentEnd);
+
                 updatedList.appendByMoving(foundIt);
             }
         } else {
 #if ENABLE(ASSERT)
             if (RuntimeEnabledFeatures::slimmingPaintUnderInvalidationCheckingEnabled())
                 checkCachedDisplayItemIsUnchanged(newDisplayItem, displayItemIndicesByClient);
             else
                 ASSERT(!DisplayItem::isDrawingType(newDisplayItem.type()) || newDisplayItem.skippedCache() || !clientCacheIsValid(newDisplayItem.client()));
 #endif // ENABLE(ASSERT)
             updatedList.appendByMoving(newIt);
@@ skipping 186 matching lines @@
 
 void DisplayItemList::replay(GraphicsContext& context) const
 {
     TRACE_EVENT0("blink,benchmark", "DisplayItemList::replay");
     ASSERT(m_newDisplayItems.isEmpty());
     for (auto& displayItem : m_currentDisplayItems)
         displayItem.replay(context);
 }
 
 } // namespace blink