Add a release assert that we found a cached item.

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 f5782e990fc17e22d7e54464599c6c4d5d24fa20..b7b083eb3bb888dbf37fff4330ad915223035f09 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -260,8 +260,12 @@ void DisplayItemList::commitNewDisplayItems()
                 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 {