TimelinePanel: convert top pane into split view.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/156353004

git-svn-id: svn://svn.chromium.org/blink/trunk@166610 bbb929c8-8fbe-4397-9dbb-9b2b20218538