Timeline: populate frame model from trace events, if tracing experiment is on

This switches the frame model to using trace events on the front-end, provided
"tracing mode" experiment is on. Note the frames still miss colored categories
filling, as we lack the categoris mapping for trace events yet.

BUG=361045

Review URL: https://codereview.chromium.org/253263005

git-svn-id: svn://svn.chromium.org/blink/trunk@173322 bbb929c8-8fbe-4397-9dbb-9b2b20218538