Reviewed by Darin Adler.

        Move UString::from(double) implementation to new
        WTF::doubleToStringInJavaScriptFormat(), and expose it because WebCore
        code will use it.
        https://bugs.webkit.org/show_bug.cgi?id=31330

        - Introduce new function createRep(const char*, unsigned) and
          UString::UString(const char*, unsigned) to reduce 2 calls to strlen().
        - Fix a bug that dtoa() doesn't update *rve if the input value is NaN
          or Infinity.

        No new tests because this doesn't change the behavior.

        * JavaScriptCore.exp:
        * JavaScriptCore.vcproj/JavaScriptCore/JavaScriptCore.def:
        * runtime/UString.cpp:
        (JSC::createRep):
        (JSC::UString::UString):
        (JSC::UString::from): Move the code to doubleToStringInJavaScriptFormat().
        * runtime/UString.h:
        * wtf/dtoa.cpp:
        (WTF::dtoa): Fix a bug about rve.
        (WTF::append): A helper for doubleToStringInJavaScriptFormat().
        (WTF::doubleToStringInJavaScriptFormat): Move the code from UString::from(double).
        * wtf/dtoa.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@51168 268f45cc-cd09-0410-ab3c-d52691b4dbfc