DevTools: Bring back inspector close button

BUG=313158
R=pfeldman@chromium.org, pfeldman

Review URL: https://codereview.chromium.org/51103010

git-svn-id: svn://svn.chromium.org/blink/trunk@160951 bbb929c8-8fbe-4397-9dbb-9b2b20218538