Add option to track when we're showing blankness during fast scrolling

Summary:
If tracking is enabled and the sampling check passes on a scroll or layout event,
we compare the scroll offset to the layout of the rendered items. If the items don't cover
the visible area of the list, we fire an `onFillRateExceeded` call with relevant stats for
logging the event through an analytics pipeline.

The measurement methodology is a little jank because everything is async, but it seems directionally
useful for getting ballpark numbers, catching regressions, and tracking improvements.

Benchmark testing shows a ~2014 MotoX starts hitting the fill rate limit at about 2500 px / sec,
which is pretty fast scrolling.

This also reworks our frame rate stuff so we can use a shared `SceneTracking` thing and track blankness
globally.

Reviewed By: bvaughn

Differential Revision: D4806867

fbshipit-source-id: 119bf177463c8c3aa51fa13d1a9d03b1a96042aa

Libraries/Utilities/SceneTracker.js

@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule SceneTracker
+ * @flow
+ */
+
+'use strict';
+
+type Scene = {name: string};
+
+let _listeners: Array<(scene: Scene) => void> = [];
+
+let _activeScene = {name: 'default'};
+
+const SceneTracker = {
+  setActiveScene(scene: Scene) {
+    _activeScene = scene;
+    _listeners.forEach((listener) => listener(_activeScene));
+  },
+
+  getActiveScene(): Scene {
+    return _activeScene;
+  },
+
+  addActiveSceneChangedListener(callback: (scene: Scene) => void): {remove: () => void} {
+    _listeners.push(callback);
+    return {
+      remove: () => {
+        _listeners = _listeners.filter((listener) => callback !== listener);
+      },
+    };
+  },
+};
+
+module.exports = SceneTracker;