DevTools: [CodeMirror] do not insert pair of quotes inside string literal

The patch implements a feature-request to not insert a pair of matching
quotes if the cursor is inside a string token.

R=apavlov, vsevik@chromium.org
BUG=411238

Review URL: https://codereview.chromium.org/590113003

git-svn-id: svn://svn.chromium.org/blink/trunk@182431 bbb929c8-8fbe-4397-9dbb-9b2b20218538