Object.prototype.toString() should use cached strings for null/undefined.
<https://webkit.org/b/133261>

Normally, when calling Object.prototype.toString() on a regular object,
we'd cache the result of the stringification on the object's structure,
making repeated calls fast.

For null and undefined, we were not as smart. We'd instead construct a
new string with either "[object Null]" or "[object Undefined]" each time.

This was exposed by Dromaeo's JS library tests, where some prototype.js
subtests generate millions of strings this way.

This patch adds two VM-permanent cached strings to the SmallStrings.
Looks like ~10% speed-up on Dromaeo/jslib-traverse-prototype.html

Reviewed by Darin Adler.

* runtime/ObjectPrototype.cpp:
(JSC::objectProtoFuncToString):
* runtime/SmallStrings.cpp:
(JSC::SmallStrings::SmallStrings):
(JSC::SmallStrings::initializeCommonStrings):
(JSC::SmallStrings::visitStrongReferences):
* runtime/SmallStrings.h:
(JSC::SmallStrings::nullObjectString):
(JSC::SmallStrings::undefinedObjectString):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@169316 268f45cc-cd09-0410-ab3c-d52691b4dbfc