Update timeStampSeconds to correspond to the code

public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 167 matching lines @@
     // expected to stick. If this axis is set to Free, then scrolling is not
     // stuck to any axis.
     enum RailsMode {
         RailsModeFree       = 0,
         RailsModeHorizontal = 1,
         RailsModeVertical   = 2,
     };
 
     static const int InputModifiers = ShiftKey | ControlKey | AltKey | MetaKey;
 
-    double timeStampSeconds; // Seconds since epoch.
+    double timeStampSeconds; // Seconds since platform start with microsecond resolution.
     unsigned size; // The size of this structure, for serialization.
     Type type;
     int modifiers;
 
     // Returns true if the WebInputEvent |type| is a mouse event.
     static bool isMouseEventType(int type)
     {
         return MouseTypeFirst <= type && type <= MouseTypeLast;
     }
 
@@ skipping 355 matching lines @@
         , uniqueTouchEventId(0)
     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif