Timeline: reorder methods in TimelinePanel.

no functional changes

BUG=none
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/113843002

git-svn-id: svn://svn.chromium.org/blink/trunk@163771 bbb929c8-8fbe-4397-9dbb-9b2b20218538