Create Tracing{Timeline}Model in TimelinePanel constructor

start/stop recording logic for trace events moved into TracingTimelineModel.

BUG=361045
R=caseq@chromium.org

Review URL: https://codereview.chromium.org/317273004

git-svn-id: svn://svn.chromium.org/blink/trunk@175649 bbb929c8-8fbe-4397-9dbb-9b2b20218538