No test cases needed.

        Implemented a version of StringImpl::append that takes a UChar* and 
        a length.

        * platform/StringImpl.cpp:
        (WebCore::StringImpl::append):
        (WebCore::StringImpl::insert): Call new append instead of making a 
        new StringImpl.
        * platform/StringImpl.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@16759 268f45cc-cd09-0410-ab3c-d52691b4dbfc