Code:  static void markPointer(Visitor* visitor, HeapObjectHeader* header) {
  ASSERT(header->checkHeader());
   const GCInfo* gcInfo = ThreadHeap::gcInfo(header->gcInfoIndex());
   if (gcInfo->hasVTable() && !vTableInitialized(header->payload())) {
    visitor->markHeaderNoTracing(header);
    ASSERT(isUninitializedMemory(header->payload(), header->payloadSize()));
  } else {
    visitor->markHeader(header, gcInfo->m_trace);
  }
}

Vulnerability Type: Overflow 
CWE ID: CWE-119
Summary: Inline metadata in GarbageCollection in Google Chrome prior to 66.0.3359.117 allowed a remote attacker to potentially exploit heap corruption via a crafted HTML page.
Commit Message: Call HeapObjectHeader::checkHeader solely for its side-effect.

This requires changing its signature. This is a preliminary stage to making it
private.

BUG=633030

Review-Url: https://codereview.chromium.org/2698673003
Cr-Commit-Position: refs/heads/master@{#460489}