DevTools: resize cursor is missing on Timeline overview.

When hovering a selection window curtain on the overview pane in Timeline,
cursor should turn ew-resize. But it is not after r163547

Review URL: https://codereview.chromium.org/112853002

git-svn-id: svn://svn.chromium.org/blink/trunk@163716 bbb929c8-8fbe-4397-9dbb-9b2b20218538