DevTools: switch Timeline frontend into buffered mode and remove the corresponding experiment.

All the tests were adjusted to the buffered mode.
Console.timeline also records timeline in buffered mode.

BUG=361045
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/399043002

git-svn-id: svn://svn.chromium.org/blink/trunk@179113 bbb929c8-8fbe-4397-9dbb-9b2b20218538