Code:  void HeapObjectHeader::zapMagic() {
  ASSERT(checkHeader());
   m_magic = zappedMagic;
 }

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}