TimelinePanel: remove overviewPanel member from TimelineView.

Drive by fix: create one overviewControl per one TimelineView.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/141143005

git-svn-id: svn://svn.chromium.org/blink/trunk@165304 bbb929c8-8fbe-4397-9dbb-9b2b20218538