input/textarea: Count text length for maxLength check with the standard way.

We counted use-input text in grapheme cluster unit. i.e. A letter +
combining characters are counted as '1,' and a surrogate pair is counted
as '1.' According to the standard and other browsers, we shoudl count
them in UTF-16 code point unit.

BUG=148852

Review URL: https://chromiumcodereview.appspot.com/22590003

git-svn-id: svn://svn.chromium.org/blink/trunk@155803 bbb929c8-8fbe-4397-9dbb-9b2b20218538