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

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