Remove some of timeline experiments

This change removes "Timeline tracing mode" and "Timeline flame chart" experiments. The latter is merged into "Timeline on trace events".

BUG=361045

Review URL: https://codereview.chromium.org/396803008

git-svn-id: svn://svn.chromium.org/blink/trunk@178376 bbb929c8-8fbe-4397-9dbb-9b2b20218538