Code: size_t NormalPage::objectPayloadSizeForTesting() {
  size_t objectPayloadSize = 0;
  Address headerAddress = payload();
  markAsSwept();
  ASSERT(headerAddress != payloadEnd());
  do {
     HeapObjectHeader* header =
         reinterpret_cast<HeapObjectHeader*>(headerAddress);
     if (!header->isFree()) {
      ASSERT(header->checkHeader());
       objectPayloadSize += header->payloadSize();
     }
     ASSERT(header->size() < blinkPagePayloadSize());
    headerAddress += header->size();
    ASSERT(headerAddress <= payloadEnd());
  } while (headerAddress < payloadEnd());
  return objectPayloadSize;
}

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}