Oilapn: Fix for ephemeron iteration callback called after the table i…

…s freed

Check if table_ exists because the table may be freed after the callback has been registered when incremental marking is enabled.

Bug: 757440
Change-Id: I677ae294f5d66f539ea6e68ddab29053aaf6c503
Reviewed-on: https://chromium-review.googlesource.com/896913
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534023}

third_party/WebKit/Source/platform/wtf/HashTable.h

@@ -2060,7 +2060,10 @@ struct WeakProcessingHashTableHelper<kWeakHandlingInCollections,
   static void EphemeronIteration(typename Allocator::Visitor* visitor,
                                  void* closure) {
     HashTableType* table = reinterpret_cast<HashTableType*>(closure);
-    DCHECK(table->table_);
+    // During incremental marking, the table may be freed after the callback has
+    // been registered.
+    if (!table->table_)
+      return;
     // Check the hash table for elements that we now know will not be
     // removed by weak processing. Those elements need to have their strong
     // pointers traced.