DevTools: Timeline: prepare for making timeline filters optional.

Timeline has hardcoded set of filters.
We are going to support flame chart which has no filters at the first glance.
So we should be ready for this.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/101653003

git-svn-id: svn://svn.chromium.org/blink/trunk@163436 bbb929c8-8fbe-4397-9dbb-9b2b20218538