Capturing Dev Tools Timeline hangs at fetching the result for some pages

[pmeenan]

It might be possible to improve this in the future and parse the trace events one at a time instead of loading the whole file but not currently (the file need to be parsed because the timeline is a subset of all trace events and is a slightly different file format).

Recent changes to the mobile agents write the events out one per line to reduce the memory overhead when parsing so once the change is also made to the desktop agents we can also add the code to the timeline downloading code to parse the file one line at a time (falling back to full-file for older tests).