DevTools: Resource Timing Table: adjust UI and cleanup code.

1) Fix vertical line alignment
2) Cleanup code
3) Add JSDocs

BUG=

Review URL: https://codereview.chromium.org/481863002

git-svn-id: svn://svn.chromium.org/blink/trunk@180529 bbb929c8-8fbe-4397-9dbb-9b2b20218538