DevTools: move timeline on tracing out of experimental

This patch makes Timeline panel use tracing backend by default. Should anything go wrong, the deprecated implementation can be reenabled using experimental setting. The setting will be removed along with the deprecated implementation once we collect enough feedback on the new implementation.

BUG=399531,361045

Review URL: https://codereview.chromium.org/597843002

git-svn-id: svn://svn.chromium.org/blink/trunk@183038 bbb929c8-8fbe-4397-9dbb-9b2b20218538