Timeline: keep separate copy of TimelineView for each view mode.

refactoring: Now we could eliminate modeChanged method and put all its
content into constructor because view doesn't change mode on the fly.
Another part of modeChanged method was moved to wasShown/willHide pair of methods.

BUG=
R=alph@chromium.org

Review URL: https://codereview.chromium.org/119083005

git-svn-id: svn://svn.chromium.org/blink/trunk@164772 bbb929c8-8fbe-4397-9dbb-9b2b20218538