DevTools: add title to computed fonts section

This patch adds a title to computed fonts section. Note that it also
removes computed css font value from it. See bug for screenshot
related to this UI change.

BUG=315118

Review URL: https://codereview.chromium.org/59993003

git-svn-id: svn://svn.chromium.org/blink/trunk@161371 bbb929c8-8fbe-4397-9dbb-9b2b20218538