DevTools: Timeline: move overview control management out of TimelineOverviewPanel

TimelinePanel manages presentation mode, so it has to assign proper Overview to TimelineOverviewPanel.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/100483005

git-svn-id: svn://svn.chromium.org/blink/trunk@163539 bbb929c8-8fbe-4397-9dbb-9b2b20218538