handle new trace object format

[brendankenny]

Fixes #555

With https://codereview.chromium.org/2161583004 the trace object we get is slightly different, with the trace events array as a traceEvents property on the object instead of as the object itself. This was causing all -1s in the performance test as they couldn't understand the updated trace format.

On the simplest level, this PR moves artifacts.traces from something like

artifacts.traces = {
  defaultPass: {
    traceContents: [/* trace events */]
  }
}

to

artifacts.traces = {
  defaultPass: {
    traceEvents: [/* trace events */],
    metaData: { /* trace meta data */}
  }
}

and then the rest of the PR is just fixing import and export code to handle both the new and the old trace object formats.

[brendankenny]

I've tested this with Chrome from before and after the format change, so any Chrome after m52 should continue to work.

[paulirish]

cc @michaelgerakis if you rely on the string traceContents in your integration, this will break you.

[paulirish]

This is excellent. Thanks @brendankenny brendan