Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/w3c-hr-time/lib/calculate-clock-offset.js

@@ -0,0 +1,37 @@
+"use strict";
+
+// This files implements the calculation of the offset between the global monotonic clock and UNIX time. This value is
+// known as |t1| in the calculation of "time origin timestamp" in the spec. This value needs to be calculated once and
+// can be used in all subsequent Performance instances.
+//
+// However, if the clock is not fast enough, the export is undefined to signify that we should use Date.now() to get the
+// time origin timestamp with millisecond accuracy, per spec.
+
+const { getGlobalMonotonicClockMS } = require("./global-monotonic-clock");
+const clockIsAccurate = require("./clock-is-accurate");
+
+// This function assumes the clock is accurate.
+function calculateClockOffset() {
+  const start = Date.now();
+  let cur = start;
+  while (cur === start) {
+    cur = Date.now();
+  }
+
+  // At this point |cur| "just" became equal to the next millisecond -- the unseen digits after |cur| are approximately
+  // all 0, and |cur| is the closest to the actual value of the UNIX time. Now, get the current global monotonic clock
+  // value and do the remaining calculations.
+
+  return cur - getGlobalMonotonicClockMS();
+}
+
+if (clockIsAccurate) {
+  // Warm up the function.
+  calculateClockOffset();
+  calculateClockOffset();
+  calculateClockOffset();
+
+  module.exports = calculateClockOffset;
+} else {
+  module.exports = undefined;
+}