findFirstGrammarDetail doesn't need to be exposed

Make TextCheckingHelper::findFirstGrammarDetail private.
Additionaly, remove badGrammarPhraseLength param, it's not used any longer
and add mising const modifier.

WebKit merge r153512.

Review URL: https://chromiumcodereview.appspot.com/22312006

git-svn-id: svn://svn.chromium.org/blink/trunk@155716 bbb929c8-8fbe-4397-9dbb-9b2b20218538