Add an experiment for using trace events as Timeline backend

This patch adds initial support for creating flame chart timeline view based on devtools trace events. It misses some features compared to the current flame chart mode:
- title formatting matching one in the current Timeline
- warnings marker
- details for selected entry
I'm going to address them one by one in next changes. After that we will build old timeline grid view based on the trace events.

BUG=361045

Review URL: https://codereview.chromium.org/272553002

git-svn-id: svn://svn.chromium.org/blink/trunk@173512 bbb929c8-8fbe-4397-9dbb-9b2b20218538