Change ui::Event::time_stamp from TimeDelta to TimeTicks

ui/events/base_event_utils.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_EVENTS_BASE_EVENT_UTILS_H_
 #define UI_EVENTS_BASE_EVENT_UTILS_H_
 
 #include <stdint.h>
 
 #include "ui/events/events_base_export.h"
 
+namespace base {
+class TimeTicks;
+}
+
 // Common functions to be used for all platforms.
 namespace ui {
 
 // Generate an unique identifier for events.
 EVENTS_BASE_EXPORT uint32_t GetNextTouchEventId();
 
 // Checks if |flags| contains system key modifiers.
 EVENTS_BASE_EXPORT bool IsSystemKeyModifier(int flags);
 
+EVENTS_BASE_EXPORT double EventTimeStampToSeconds(base::TimeTicks time_stamp);

[miu, 2016/05/16 20:05:35]

Please add some comments to explain the usage of these:

1. These should rarely be used (i.e., only when interfacing with platform code
that does not use Chromium's base::TimeXXX type system). Otherwise, client code
should maintain the time values in TimeTicks form.

2. The time origin of the doubles is not fixed between runs of the application
(and is certainly not any sort of UNIX epoch, or some such).

In fact, do you absolutely need these? And, if you do, can they be made private
or otherwise placed in an "internal" namespace to discourage outside use?

[majidvp, 2016/05/21 01:38:00]

I will add a comment. As you mentioned these should only
be used when interfacing with platform code. I am planing
to eventually get rid of that usecase as well.
These are being used by mojo/converter as well so I cannot make them private. Do
you still think it is useful to make them internal
given that  they will be used outside ui/events?

[miu, 2016/05/25 01:42:31]

IIRC, mojo includes a serializer for the base::TimeTicks type. If not, it
should. ;)

My main concern here is that external code will attempt to interpret the meaning
of the double values (e.g., assuming some specific time origin, or relating the
value to other clocks, or to some calendar point-in-time). While this is not the
case in your change here--since you're only using these to translate to/from
native platform UI toolkit values--it does set a possible starting point for
unintentional "code rot" in the future.

I'll leave it up to you and ui/events/OWNERS to decide whether it's acceptable
for these functions to be publicly visible like they are. You may simply want to
comment them with a "WARNING: Only for native platform value translation use
within ui/events code!"

[majidvp, 2016/05/27 14:34:15]

Mojo has convertor for TimeTicks but we are converting TimeTicks to seconds.
It has a utility function for that as well but I intentionally removed in favor
of 
consolidating all such event time conversions in these pair of functions.
I added a WARNING to the comment.

+EVENTS_BASE_EXPORT base::TimeTicks EventTimeStampFromSeconds(
+    double time_stamp_seconds);
+
 }  // namespace ui
 
 #endif  // UI_EVENTS_BASE_EVENT_UTILS_H_