https://bugs.webkit.org/show_bug.cgi?id=108253

Reviewed by Mark Hahnenberg.

When accessing methodTable() we now always make sure that our
structure _could_ be valid.  Added a separate method to get a
classes methodTable during destruction as it's not possible to
validate the structure at that point.  This separation might
also make it possible to improve the performance of methodTable
access more generally in future.

* heap/MarkedBlock.cpp:
(JSC::MarkedBlock::callDestructor):
* runtime/JSCell.h:
(JSCell):
* runtime/JSCellInlines.h:
(JSC::JSCell::methodTableForDestruction):
(JSC):
(JSC::JSCell::methodTable):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141190 268f45cc-cd09-0410-ab3c-d52691b4dbfc