Zero out CopiedBlocks on initialization

Zero out CopiedBlocks on initialization
https://bugs.webkit.org/show_bug.cgi?id=79199

Reviewed by Filip Pizlo.

Made CopyBlocks zero their payloads during construction. This allows
JSArray to avoid having to manually clear its backing store upon allocation
and also alleviates any future pain with regard to the garbage collector trying
to mark what it thinks are values in what is actually uninitialized memory.

* heap/CopiedBlock.h:
(JSC::CopiedBlock::CopiedBlock):
* runtime/JSArray.cpp:
(JSC::JSArray::finishCreation):
(JSC::JSArray::tryFinishCreationUninitialized):
(JSC::JSArray::increaseVectorLength):
(JSC::JSArray::unshiftCountSlowCase):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108716 268f45cc-cd09-0410-ab3c-d52691b4dbfc