SkTime updates

1) Use steady_clock instead of high_resolution_clock. If we don't have a
guarantee of monotonicity, it's pretty much useless for timing things.

2) Implement Mac/iOS with <chrono> too.  This was waiting on C++11 library support.

Both high_resolution_clock and steady_clock are (still) busted on MSVC 2013,
so no change there.

BUG=skia:

Review URL: https://codereview.chromium.org/1521293002
1 file changed