[DevTools] Fix MediaQueryInspector not working in toolbox.

BUG=none
TBR=apavlov

Review URL: https://codereview.chromium.org/433443004

git-svn-id: svn://svn.chromium.org/blink/trunk@179266 bbb929c8-8fbe-4397-9dbb-9b2b20218538