fixing tracking metrics

Summary:
- fixing dropped frames calculation
- fixing plugin activation time

Reviewed By: passy

Differential Revision: D9301759

fbshipit-source-id: 872e4d2edcafdbc67668f3d0b713dfbf55f068f5

src/dispatcher/tracking.js

@@ -18,8 +18,9 @@ export default (store: Store, logger: Logger) => {
   ) {
     const now = performance.now();
     requestAnimationFrame(() => droppedFrameDetection(now, isWindowFocused));
-    if (isWindowFocused()) {
-      droppedFrames += Math.max(0, Math.round(now - past / (1000 / 60) - 1));
+    const dropped = Math.round((now - past) / (1000 / 60) - 1);
+    if (dropped > 0 && isWindowFocused()) {
+      droppedFrames += dropped;
     }
   }