Make HeapObjectHeader::checkHeader private.

The function is properly private; callers should not have to 'know' when it's
necessary and when it isn't. It's the object's job to enforce its own internal
invariants.

BUG=633030

Review-Url: https://codereview.chromium.org/2786843002
Cr-Commit-Position: refs/heads/master@{#461220}
8 files changed