message: fix(apm): Use Performance API for timings when available
  Instead of downgrading to use the dumb performanceFallback, in the
  absence of a timeOrigin we can use INITIAL_TIME.
  This should give better timings in Safari Web Workers.
diff: @@ -378,19 +378,11 @@ export const crossPlatformPerformance: Pick<Performance, 'now' | 'timeOrigin'> =
  // is not as widely supported. Namely, performance.timeOrigin is undefined in Safari as of writing.
  // tslint:disable-next-line:strict-type-predicates
  if (performance.timeOrigin === undefined) {
  - // For webworkers it could mean we don't have performance.timing then we fallback
  - // tslint:disable-next-line:deprecation
  - if (!performance.timing) {
  - return performanceFallback;
  - }
  - // tslint:disable-next-line:deprecation
  - if (!performance.timing.navigationStart) {
  - return performanceFallback;
  - }
  -
  + // As of writing, performance.timing is not available in Web Workers in mainstream browsers, so it is not always a
  + // valid fallback. In the absence of a initial time provided by the browser, fallback to INITIAL_TIME.
  // @ts-ignore
  // tslint:disable-next-line:deprecation
  - performance.timeOrigin = performance.timing.navigationStart;
  + performance.timeOrigin = (performance.timing && performance.timing.navigationStart) || INITIAL_TIME;
  }
  }
  