ThreadSafeRefCountedGarbageCollected should use makeKeepAlive

Similar to RefCountedGarbageCollected, ThreadSafeRefCountedGarbageCollected
should use a helper method to create a self-keep-alive handle.
This change is to make it easier to blacklist a bad cast detected in UBSan.

BUG=405345
NOTRY=true

Review URL: https://codereview.chromium.org/489353003

git-svn-id: svn://svn.chromium.org/blink/trunk@180756 bbb929c8-8fbe-4397-9dbb-9b2b20218538