DevTools: fix Timeline tree item border color

After r182835 it became black. This fixes it to be darker than background of corresponding icon.

BUG=None
NOTRY=True

Review URL: https://codereview.chromium.org/621823002

git-svn-id: svn://svn.chromium.org/blink/trunk@183050 bbb929c8-8fbe-4397-9dbb-9b2b20218538