Teach browsertime to collect Gecko internal PerfStats

[ncalexan]

Over in Bug 1553254, a new micro-benchmark Gecko API is being implemented. This ticket tracks teaching browsertime to enable that API and to collect the resulting data.

[ncalexan]

The WIP I have is in ncalexan@687bf81.

[hawkinsw]

The WIP (very, very early!) is in a5ab734

[hawkinsw]

A new-style script would look something like

({
  'requiredPrivs': ['privilege'],
  'function': (function() {
  const measures = [];
  const marks = [];

  if (window.performance && window.performance.getEntriesByType) {
    const myMarks = Array.prototype.slice.call(
      window.performance.getEntriesByType('mark')
    );

    for (const mark of myMarks) {
      marks.push({
        name: mark.name,
        startTime: mark.startTime
      });
    }

    const myMeasures = Array.prototype.slice.call(
      window.performance.getEntriesByType('measure')
    );

    for (const measure of myMeasures) {
      measures.push({
        name: measure.name,
        duration: measure.duration,
        startTime: measure.startTime
      });
    }
  }

  return {
    marks: marks,
    measures: measures
  };
})
})