TimelinePanel: make overview split not sizable.

Followup patch after r166610

BUG=326121
TBR=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/156693002

git-svn-id: svn://svn.chromium.org/blink/trunk@166617 bbb929c8-8fbe-4397-9dbb-9b2b20218538