DevTools: [CodeMirror] fix plus-one bug in omitting quotes in string literals

The token value was determined erroneously, the patch fixes it.

R=apavlov
BUG=420268

Review URL: https://codereview.chromium.org/623153002

git-svn-id: svn://svn.chromium.org/blink/trunk@183228 bbb929c8-8fbe-4397-9dbb-9b2b20218538