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Fix import for Chrome 69, support leading idle time before first call (#160)

This PR fixes #159, and also fixes various small things about how profiles were imported for previous versions of Chrome & for Firefox.

The Chrome 69 format splits profiles across several [Trace Event Format](https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview) events. There are two relevant events: "Profile" and "ProfileChunk". At first read through a profile, it seems like profiles are incorrectly terminated, but it seems like the cause of that is that, for whatever reason, events in the event log are not always sorted in chronological order. If sorted chronologically, then the event sequence can be parsed sensibly.

In the process of looking at this information, I also discovered that speedscope's chrome importer was incorrectly interpreting the value of the first element in `timeDeltas` array. It's intended to be the elapsed time since the start of the profile, not the time between the first pair of samples. This changes the weight attributed to the first sample.

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