DevTools: ability to click close button in docked frontend multiple times

This patch makes close button in docked devtools frontend clickable
multiple times which might be useful whenever beforeunload event handler
will be added to frontend.

BUG=

Review URL: https://codereview.chromium.org/72823002

git-svn-id: svn://svn.chromium.org/blink/trunk@162062 bbb929c8-8fbe-4397-9dbb-9b2b20218538