ASSERTION FAILED: m_scriptExecutionContext->isContextThread() seen with LayoutTest...

ASSERTION FAILED: m_scriptExecutionContext->isContextThread() seen with LayoutTest crypto/workers/crypto-random-values-limits-worker.html
https://bugs.webkit.org/show_bug.cgi?id=171462
<rdar://problem/31906859>

Reviewed by Brent Fulgham.

Covered by existing tests.

* workers/WorkerGlobalScope.cpp:
(WebCore::WorkerGlobalScope::~WorkerGlobalScope):
Clear Crypto early in destruction since its ContextDestructionObserver
destruction makes checks about the WorkerThread.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@216133 268f45cc-cd09-0410-ab3c-d52691b4dbfc