input.validity.tooLong check should count the value in UTF-16 code point unit.

This is a leftover of r155803.

BUG=148852
TEST=automated.

Review URL: https://chromiumcodereview.appspot.com/24107004

git-svn-id: svn://svn.chromium.org/blink/trunk@158018 bbb929c8-8fbe-4397-9dbb-9b2b20218538