Implement RuntimeScheduler::now

Summary:
Changelog: [internal]

Implement RuntimeScheduler::now and exposite it to JavaScript. JavaScripts expects miliseconds and is using `performance.now()` call to get this value.

More about `performance.now()`: https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
JavaScript implementation in Scheduler: https://github.com/facebook/react/blob/master/packages/scheduler/src/forks/SchedulerNoDOM.js#L71-L82

Reviewed By: JoshuaGross

Differential Revision: D27675512

fbshipit-source-id: bce5ac700e71b1722cd280dfbdd15e141783a3a5

ReactCommon/react/renderer/runtimescheduler/RuntimeScheduler.cpp

@@ -30,4 +30,8 @@ bool RuntimeScheduler::getShouldYield() const {
   return shouldYield_;
 }
 
+RuntimeSchedulerClock::time_point RuntimeScheduler::now() const {
+  return RuntimeSchedulerClock::now();
+}
+
 } // namespace facebook::react

ReactCommon/react/renderer/runtimescheduler/RuntimeScheduler.h

@@ -10,11 +10,17 @@
 #include <ReactCommon/RuntimeExecutor.h>
 #include <react/renderer/runtimescheduler/Task.h>
 #include <atomic>
+#include <chrono>
 #include <memory>
 #include <queue>
 
 namespace facebook::react {
 
+/*
+ * Represents a monotonic clock suitable for measuring intervals.
+ */
+using RuntimeSchedulerClock = std::chrono::steady_clock;
+
 class RuntimeScheduler final {
  public:
   RuntimeScheduler(RuntimeExecutor const &runtimeExecutor);
@@ -25,6 +31,8 @@ class RuntimeScheduler final {
 
   bool getShouldYield() const;
 
+  RuntimeSchedulerClock::time_point now() const;
+
  private:
   mutable std::priority_queue<
       std::shared_ptr<Task>,

ReactCommon/react/renderer/runtimescheduler/RuntimeSchedulerBinding.cpp

@@ -10,6 +10,7 @@
 #include "primitives.h"
 
 #include <react/debug/react_native_assert.h>
+#include <chrono>
 #include <memory>
 
 namespace facebook::react {
@@ -115,6 +116,24 @@ jsi::Value RuntimeSchedulerBinding::get(
         });
   }
 
+  if (propertyName == "unstable_now") {
+    return jsi::Function::createFromHostFunction(
+        runtime,
+        name,
+        0,
+        [this](
+            jsi::Runtime &,
+            jsi::Value const &,
+            jsi::Value const *,
+            size_t) noexcept -> jsi::Value {
+          auto now = runtimeScheduler_->now();
+          auto asDouble =
+              std::chrono::duration<double, std::milli>(now.time_since_epoch())
+                  .count();
+          return jsi::Value(asDouble);
+        });
+  }
+
   if (propertyName == "unstable_ImmediatePriority") {
     return jsi::Value(runtime, serialize(SchedulerPriority::ImmediatePriority));
   }