Clear the safepoint marker only after we left the safepoint.

We should not change any GC related parts of the ThreadState while we are inside the safepoint region because GC can be in progress and it can be accesing them. In this case SafePointBarrier will park the current thread until GC is over. It is safe to mutate ThreadState after we successfully left safepoint as this means that there is no GC in progress.

BUG=342856

Review URL: https://codereview.chromium.org/162633003

git-svn-id: svn://svn.chromium.org/blink/trunk@167127 bbb929c8-8fbe-4397-9dbb-9b2b20218538