Remove String::adopt(Vector<UChar>)

Adopting a Vector<UChar> isn't an efficient way to build a string. There are
only two callers, so we should remove this API before we grow more. I've moved
the callers over to StringBuilder and StringBuffer. There was one caller in
EventSource that I moved to copying the characters rather than adopting them.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/21262003

git-svn-id: svn://svn.chromium.org/blink/trunk@155225 bbb929c8-8fbe-4397-9dbb-9b2b20218538