Independent of start()/stop()/getElapsedTime() functionality,
getAbsoluteTime() is added to return the time regardless of whether the
timer is running.

Bug: angleproject:2908
Change-Id: I056aeb6eddfba8757a139934c1cf68e00e860de6
Reviewed-on: https://chromium-review.googlesource.com/c/1296952
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>