DevTools: [CodeMirror] Ignore numbers in autocompletion dictionary

This patch adds a functionality to ignore digits so that they are not
added to the autocompletion dictionary.

BUG=

Review URL: https://chromiumcodereview.appspot.com/19806003

git-svn-id: svn://svn.chromium.org/blink/trunk@154644 bbb929c8-8fbe-4397-9dbb-9b2b20218538