A TimelineTask represents an asynchronous track of operations on the timeline.
The old API allowed multiple isolates to emit events simultaneously on the same task or to start / finish events in any order. This isn't supported by our UI and probably many other tools that consume trace-event.
With this change, operations within a TimelineTask must complete in the reverse order of their start calls (aka a stack). You can pass a TimelineTask from one isolate to another but you must first complete all open operations before passing it.
Example:
var task = new TimelineTask();
task.start('Some phase');
await someFuture;
task.finish();
Also: Add support for instant events.
R=rmacnak@google.com
Review URL: https://codereview.chromium.org/1412183008 .
John McCutchan