TimelinePanel: reduce number of resize events on initial show.

It had 3 resize events in order of constructing views.
Now it has only 2 resize events.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/143353002

git-svn-id: svn://svn.chromium.org/blink/trunk@165405 bbb929c8-8fbe-4397-9dbb-9b2b20218538