Initialize min/max record time to 0 instead of -1 or null

Source/devtools/front_end/timeline/TracingModel.js

Index: Source/devtools/front_end/timeline/TracingModel.js
diff --git a/Source/devtools/front_end/timeline/TracingModel.js b/Source/devtools/front_end/timeline/TracingModel.js
index 9dd7a0ec899249d8788659fee97459b25fc39efb..7a25de046652f3f1262815d04c2ffc148582db76 100644
--- a/Source/devtools/front_end/timeline/TracingModel.js
+++ b/Source/devtools/front_end/timeline/TracingModel.js
@@ -164,8 +164,8 @@ WebInspector.TracingModel.prototype = {
     reset: function()
     {
         this._processById = {};
-        this._minimumRecordTime = null;
-        this._maximumRecordTime = null;
+        this._minimumRecordTime = 0;
+        this._maximumRecordTime = 0;
         this._sessionId = null;
         this._devtoolsMetadataEvents = [];
     },
@@ -218,7 +218,7 @@ WebInspector.TracingModel.prototype = {
     },
 
     /**
-     * @return {?number}
+     * @return {number}
      */
     minimumRecordTime: function()
     {
@@ -226,7 +226,7 @@ WebInspector.TracingModel.prototype = {
     },
 
     /**
-     * @return {?number}
+     * @return {number}
      */
     maximumRecordTime: function()
     {