DevTools: Wrong position of the remove watch expression button.

Wrong position of the "remove watch expression" button when docked to bottom.
Probably, regressed when we switched to flex.

R=vsevik, pfeldman

Review URL: https://codereview.chromium.org/110243002

git-svn-id: svn://svn.chromium.org/blink/trunk@163532 bbb929c8-8fbe-4397-9dbb-9b2b20218538