Add GetThreadToInteractionRecordsMapsFromModel method to timeline_interaction_record.

Add GetThreadToInteractionRecordsMapsFromModel method to timeline_interaction_record.

This method creates a map from render threads to the list of interaction records
belong to that thread. This is nice since it allows us to:
1) Simplify the interface of timeline_based_metrics.AddResults method
2) Allows us to take measurements of interaction records belong to different threads.
A potential use case is timeline_based_measurement can work for a multi tab test.
3) This allows us to remove the tab.id, the map from telemetry tab objects to
render theads and the logic of issuing tab id on the timeline model, which 
allows us to get partial metrics for timeline_based_measurement in case  
trace buffer overflowed.
4) Profit!!

BUG=

[nednguyen, 2014-08-04 18:31:00]

ptal to the changes in timeline_interaction_record, timeline_based_metric, 
web_perf/metrics/smoothness.py, and perf/metrics/timeline.py.

This is still a wip since not all the tests pass yet and I haven't remove the
tab.id, tab_to_renderer_thread mess in model yet but it would be helpful if you
can provide some sanity check to the approach.