DevTools: extract TracingManager from TracingModel

Let TracingManager handle back-end interactions while TracingModel
acts just as a container for trace events. The model will later be
moved out of sdk and this will allow introducing backing storage for
trace events (e.g. a file).

BUG=412709

Review URL: https://codereview.chromium.org/557013005

git-svn-id: svn://svn.chromium.org/blink/trunk@181741 bbb929c8-8fbe-4397-9dbb-9b2b20218538