Fixit: Split base::TimeTicks --> TimeTicks + ThreadTicks + TraceTicks

Fixit: Split base::TimeTicks --> TimeTicks + ThreadTicks + TraceTicks

TimeTicks was being overused for time values from three different clock
sources.  This change splits the class into three separate classes: The
general-purpose monotonic time (TimeTicks), the thread-local run time
(ThreadTicks), and the global system trace time (TraceTicks).

Before this change, base::Time and base::TimeTicks had identical
comparison and math operator overloads.  This change creates an internal
templated base class which all of Time, TimeTicks, ThreadTicks, and
TraceTicks subclass from; in order to de-dupe this common functionality.

Updated base/trace_event/* and a few outside-of-base uses of ThreadNow()
and NowFromSystemTraceTime() to use the new classes.  Thankfully, the
compiler is now able to use type-checking to guarantee values from
different clocks are not being mixed when doing time math.

BUG=467417

[miu, 2015-05-01 23:03:46]

Patchset #1 (id:1) has been deleted

[miu, 2015-05-04 20:57:18]

miu@chromium.org changed reviewers:
+ thestig@chromium.org

[miu, 2015-05-04 20:57:19]

thestig: PTAL at base/time/*.  If this approach looks good, I'll proceed with
rest of the review.

[Lei Zhang, 2015-05-04 22:13:30]

Generally sounds good. Would you consider doing the common functionality
de-duping as one CL, and adding the new time types as a second CL?

[miu, 2015-05-05 01:22:55]

On 2015/05/04 22:13:30, Lei Zhang wrote:
> Generally sounds good. Would you consider doing the common functionality
> de-duping as one CL, and adding the new time types as a second CL?

Will do.  I'll send them out shortly...