DevTools: Resize panel after hiding drawer.

This fixes a bug when a white strip is shown in the bottom of the editor after closing the drawer.

BUG=328836

Review URL: https://codereview.chromium.org/116393002

git-svn-id: svn://svn.chromium.org/blink/trunk@163968 bbb929c8-8fbe-4397-9dbb-9b2b20218538