Timeline: hovered line highlight goes away when cursor hovers over expanding element.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/64553008

git-svn-id: svn://svn.chromium.org/blink/trunk@162154 bbb929c8-8fbe-4397-9dbb-9b2b20218538