Revert 154802 "Don't use Vector<UChar> to build strings"

This CL broke webkit_unit_tests in Android bots.
http://build.chromium.org/p/chromium.linux/builders/Android%20Tests%20%28dbg%29/builds/12998/steps/webkit_unit_tests/logs/stdio


> Don't use Vector<UChar> to build strings
> 
> This CL removes a bunch of code sites that were using Vector<UChar> to
> construct strings. That used to be a good way to construct strings, but in the
> modern world, we have much better alternatives in the form of StringBuffer and
> StringBuilder. (You should use StringBuffer if you know exactly how much memory
> you need and you should use StringBuilder if you require a variable amount of
> memory.)
> 
> This CL removes more clients of the StringImpl pointer. There are still a few
> left, which require more study.
> 
> R=eseidel
> BUG=262320
> 
> Review URL: https://chromiumcodereview.appspot.com/20002008

TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/19982008

git-svn-id: svn://svn.chromium.org/blink/trunk@154825 bbb929c8-8fbe-4397-9dbb-9b2b20218538