[DevTools] Show border between split view sides inside a resizer.

Placing border on side elements themselves did not respect the minimum size
and therefore messed up layout down the tree.

BUG=none

Review URL: https://codereview.chromium.org/195003002

git-svn-id: svn://svn.chromium.org/blink/trunk@169152 bbb929c8-8fbe-4397-9dbb-9b2b20218538