Fix ptmt#162: use the right timestamp for RTCFrameUpdate

Author: ptmt

Diff for: React/Base/RCTFrameUpdate.m

@@ -21,8 +21,8 @@ @implementation RCTFrameUpdate
 - (instancetype)initWithTimer:(NSTimer *)timer
 {
   if ((self = [super init])) {
-    _timestamp = timer.timeInterval;
-    _deltaTime = timer.tolerance; // TODO: real duration
+    _timestamp = timer.fireDate.timeIntervalSince1970;
+    _deltaTime = timer.timeInterval;
   }
   return self;
 }