DevTools: get paint profiler out of experiment

This removes paint profiler experiment. The paint profiler
is still only enabled for trace-based timeline, which is still
experimental. Paint profiler was originally hidden behind an
experiment of its own due to stability problems with processing
large traces, which are now fixed.

Review URL: https://codereview.chromium.org/596213002

git-svn-id: svn://svn.chromium.org/blink/trunk@182680 bbb929c8-8fbe-4397-9dbb-9b2b20218538