http://www.wunderground.com/US/CA/Hayward.html causes big memory spike during page loading 
        
Reviewed by Gavin Barraclough.

Creating a substring caused the original string be flattened if it was in the rope form. This could use
significant amount of memory by reducing buffer sharing between strings.
        
Add a rope specific substring function that constructs the substring by reusing the rope fibers
instead of flattening the rope.
        
No change observed in SunSpider.

* runtime/JSString.cpp:
(JSC::JSString::substringFromRope):
* runtime/JSString.h:
(JSC::jsSubstring):
* runtime/StringPrototype.cpp:
(JSC::stringProtoFuncSubstr):
(JSC::stringProtoFuncSubstring):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@73433 268f45cc-cd09-0410-ab3c-d52691b4dbfc