Unify native performanceNow implementation across platforms

Summary:
[Changelog][Internal]

Both iOS and Android platforms are at this point using the same native implementation of `performanceNow`, based on `std::chrono` (it used to be different some time ago).

This diff unifies the implementations, so it comes from one place in C++ code for all platforms.

The context is that I am developing event timing instrumentation and need a consistent way to get current timestamp from either JS or native (C++) side. The latter is now possible via calling `JSExecutor::performanceNow()`, which is guaranteed to be the same as called from JS.

Reviewed By: christophpurrer

Differential Revision: D42267898

fbshipit-source-id: dcb592f37d6567340ea59faddbf3b6d2b8507d50

ReactCommon/cxxreact/JSExecutor.cpp

@@ -11,6 +11,8 @@
 
 #include <folly/Conv.h>
 
+#include <chrono>
+
 namespace facebook {
 namespace react {
 
@@ -23,5 +25,15 @@ std::string JSExecutor::getSyntheticBundlePath(
   return folly::to<std::string>("seg-", bundleId, ".js");
 }
 
+double JSExecutor::performanceNow() {
+  auto time = std::chrono::steady_clock::now();
+  auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(
+                      time.time_since_epoch())
+                      .count();
+
+  constexpr double NANOSECONDS_IN_MILLISECOND = 1000000.0;
+  return duration / NANOSECONDS_IN_MILLISECOND;
+}
+
 } // namespace react
 } // namespace facebook