JSPropertyNameEnumerator::visitChildren() needs to visit its m_cachedStructureID.

JSPropertyNameEnumerator::visitChildren() needs to visit its m_cachedStructureID.
https://bugs.webkit.org/show_bug.cgi?id=189124
<rdar://problem/43863605>

Reviewed by Filip Pizlo.

JSTests:

* stress/regress-189124.js: Added.

Source/JavaScriptCore:

It is assumed that the Structure for the m_cachedStructureID will remain alive
while the m_cachedStructureID is in use.  This prevents the structureID from being
re-used for a different Structure.

* runtime/JSPropertyNameEnumerator.cpp:
(JSC::JSPropertyNameEnumerator::visitChildren):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@235715 268f45cc-cd09-0410-ab3c-d52691b4dbfc