DevTools: NetworkPanel: fix timeline bar labels vertical alignment.

Fix UI regression.
See: https://codereview.chromium.org/474453002/

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/471703003

git-svn-id: svn://svn.chromium.org/blink/trunk@180273 bbb929c8-8fbe-4397-9dbb-9b2b20218538