TimelinePanel: normalize sidebarResize code.

The old code was quite complex and fragile.

BUG=326121
R=alph@chromium.org

Review URL: https://codereview.chromium.org/130043004

git-svn-id: svn://svn.chromium.org/blink/trunk@165393 bbb929c8-8fbe-4397-9dbb-9b2b20218538