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

git-svn-id: svn://svn.chromium.org/blink/trunk@154802 bbb929c8-8fbe-4397-9dbb-9b2b20218538