Added basic UI for Nestable Async Events

This CL adds basic UI support for nestable async events.
Still need to find out whether the UI looks okay. But right
now it looks like the picture in the design doc below.

Design Doc:
https://docs.google.com/document/d/1Z2uqj59UEts5IiXX78mkdU4kd6e7kE3JUKPoDK97bVs/edit?usp=sharing
The reason is that when NetLog converts to Trace event, we will pass in all params to tracing (https://code.google.com/p/chromium/codesearch#chromium/src/net/base/trace_net_log_observer.cc&l=53). However, since we concatenate arguments of the matching pair (BEGIN and END event), if the latter one has an empty params, the resulting params will be empty (because of the logic in tvcm.concatenateObjects). Instead, the desired output will be a combined params from both events.