DevTools: Clean up boilerplate code via using statement.

Extracted from https://codereview.chromium.org/74063002/

R=apavlov, loislo

Review URL: https://codereview.chromium.org/77173006

git-svn-id: svn://svn.chromium.org/blink/trunk@162366 bbb929c8-8fbe-4397-9dbb-9b2b20218538