src: TraceEventScope should mark sync duration events

According to the chrome trace event format document, works that are performed on one single thread should be traced with sync duration events. In this way, these events can be grouped under one thread and the trace event viewer can estimate the CPU usage of that thread.

click to see screen shots

Before:

Note that there are no events recorded in the JavaScriptMainThread nor WorkerThread 1.

After:

Environment events like cleanup are correctly grouped under their thread.