Event timestamp values are now consistently coming from the same clock as
base::TimeTicks. Historically this was not the case in particular on Windows
platform which is why we were using  base::TimeDelta instead.

Now that this has been addressed we should use base::TimeTicks to make it clear
that these values are expected to come from the same clock and remove some
crufty code.

The changes in the CL are pretty simple and mechanical:
- Introduce a pair of conversion functions in ui/events/base_event_utils.h to
convert between  WebInputEvent::timeStampSeconds (double) and
ui::Event::timeStamp (TimeTicks)
- TimeDelta() =>TimeTicks()
- TimeDelta::FromMilliseconds(0) => base::TimeTicks()
- TimeDelta::FromInternalValue(TimeTicks::Now().ToInternalValue())  => TimeTicks::Now()
- TimeDelta::FromMilliseconds(Time::Now().ToDoubleT() * 1000) => TimeTicks::Now()
- Time::NowFromSystemTime() - base::Time() => TimeTicks::Now()

BUG=453559

Review-Url: https://codereview.chromium.org/1975533002
Cr-Commit-Position: refs/heads/master@{#398947}