Revert of SkTime updates (patchset #2 id:20001 of https://codereview.chromium.org/1521293002/ )

Reason for revert:
linux Canary builder has no std::steady_clock.  Weird...

Original issue's description:
> 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:
>
> Committed: https://skia.googlesource.com/skia/+/6a20871e5aeaa7e61f3348694bf436af16f824b9

TBR=herb@google.com,mtklein@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1529603002

1 files changed, 15 insertions, 4 deletions

diff --git a/src/core/SkTime.cpp b/src/core/SkTime.cpp
index 16f66161a7..86a0685c70 100644
--- a/src/core/SkTime.cpp
+++ b/src/core/SkTime.cpp
@@ -36,13 +36,24 @@ void SkTime::DateTime::toISO8601(SkString* dst) const {
             return new double(1e6 / khz.QuadPart);
         });
     }
+#elif defined(__MACH__)
+    // TODO: fold into std::chrono when available?
+    #include <mach/mach_time.h>
+    SK_DECLARE_STATIC_ONCE_PTR(double, ns_per_tick);
+    double SkTime::GetNSecs() {
+        uint64_t ticks = mach_absolute_time();
+        return ticks * *ns_per_tick.get([]{
+            mach_timebase_info_data_t timebase;
+            (void)mach_timebase_info(&timebase);
+            return new double(timebase.numer * 1.0 / timebase.denom);
+        });
+    }
 #else
-    // This std::chrono code looks great on Linux, Mac, and Android,
-    // but MSVC 2013 returns mostly garbage (0ns times, etc).
-    // This is ostensibly fixed in MSVC 2015.
+    // This std::chrono code looks great on Linux and Android,
+    // but MSVC 2013 returned mostly garbage (0ns times, etc).
     #include <chrono>
     double SkTime::GetNSecs() {
-        auto now = std::chrono::steady_clock::now();
+        auto now = std::chrono::high_resolution_clock::now();
         std::chrono::duration<double, std::nano> ns = now.time_since_epoch();
         return ns.count();
     }