Fix regression - 100% CPU - Regression for Windows 7 (#5557)

Summary:
Fixes https://github.com/facebook/rocksdb/issues/5552
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5557

Differential Revision: D16266329

fbshipit-source-id: a8f6b50298a6f7c8d6c7e172bb26dd7eb6bd8a4d

port/win/env_win.cc

@@ -979,8 +979,7 @@ uint64_t WinEnvIO::NowMicros() {
     return li.QuadPart;
   }
   using namespace std::chrono;
-  return duration_cast<microseconds>(
+  return duration_cast<microseconds>(system_clock::now().time_since_epoch()).count();
-      high_resolution_clock::now().time_since_epoch()).count();
 }
 
 uint64_t WinEnvIO::NowNanos() {