TimelinePanel: do not scroll out grid header element in TimelineView.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/166893002

git-svn-id: svn://svn.chromium.org/blink/trunk@167233 bbb929c8-8fbe-4397-9dbb-9b2b20218538