Disable the thread-safe MQ parser for main thread

This reverts commit d1eea2e66e64c3c019885ad7e94933aa0fc05576.
On top of that, this CL makes sure that only the preloader uses the thread safe MQ parser. That's in order to reduce the risk of breaking content when introducing the new MQ parser.

BUG=358078

Review URL: https://codereview.chromium.org/219183002

git-svn-id: svn://svn.chromium.org/blink/trunk@170494 bbb929c8-8fbe-4397-9dbb-9b2b20218538