Timeline: extract TimelineView related code from onModeChanged method.

BUG=
R=alph@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/114063002

git-svn-id: svn://svn.chromium.org/blink/trunk@163847 bbb929c8-8fbe-4397-9dbb-9b2b20218538