Timeline: show/hide recordsCounter via styles.

TimelinePanel gets the style for the current mode.
So the UI changes its representation automatically.

BUG=none
R=alph@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/100213007

git-svn-id: svn://svn.chromium.org/blink/trunk@163784 bbb929c8-8fbe-4397-9dbb-9b2b20218538