DevTools: extract TracingManager from TracingModel, step 2

This just moves code from sdk/TracingModel.js to timeline/TracingModel.js
and renames the former to sdk/TracingManager.js. No actual code was harmed.

BUG=412709

Review URL: https://codereview.chromium.org/563463003

git-svn-id: svn://svn.chromium.org/blink/trunk@181797 bbb929c8-8fbe-4397-9dbb-9b2b20218538