Move StatTimer from src/shared/util.js to src/display/dom_utils.js

Since the `StatTimer` is not used in the worker, duplicating this code on both the main and worker sides seem completely unnecessary.

src/display/dom_utils.js

@@ -461,6 +461,66 @@ function isExternalLinkTargetSet() {
   }
 }
 
+var StatTimer = (function StatTimerClosure() {
+  function rpad(str, pad, length) {
+    while (str.length < length) {
+      str += pad;
+    }
+    return str;
+  }
+  function StatTimer() {
+    this.started = Object.create(null);
+    this.times = [];
+    this.enabled = true;
+  }
+  StatTimer.prototype = {
+    time: function StatTimer_time(name) {
+      if (!this.enabled) {
+        return;
+      }
+      if (name in this.started) {
+        warn('Timer is already running for ' + name);
+      }
+      this.started[name] = Date.now();
+    },
+    timeEnd: function StatTimer_timeEnd(name) {
+      if (!this.enabled) {
+        return;
+      }
+      if (!(name in this.started)) {
+        warn('Timer has not been started for ' + name);
+      }
+      this.times.push({
+        'name': name,
+        'start': this.started[name],
+        'end': Date.now(),
+      });
+      // Remove timer from started so it can be called again.
+      delete this.started[name];
+    },
+    toString: function StatTimer_toString() {
+      var i, ii;
+      var times = this.times;
+      var out = '';
+      // Find the longest name for padding purposes.
+      var longest = 0;
+      for (i = 0, ii = times.length; i < ii; ++i) {
+        var name = times[i]['name'];
+        if (name.length > longest) {
+          longest = name.length;
+        }
+      }
+      for (i = 0, ii = times.length; i < ii; ++i) {
+        var span = times[i];
+        var duration = span.end - span.start;
+        out += rpad(span['name'], ' ', longest) + ' ' + duration + 'ms\n';
+      }
+      return out;
+    },
+  };
+  return StatTimer;
+})();
+
 export {
   CustomStyle,
   RenderingCancelledException,
@@ -474,4 +534,5 @@ export {
   DOMCMapReaderFactory,
   DOMSVGFactory,
   SimpleXMLParser,
+  StatTimer,
 };