Remove rAF export from ReactDOMFrameScheduling (#10337)

Fiber doesn't schedule animation callbacks anymore (though it does use
the browser's requestAnimationFrame to polyfill requestIdleCallback).

This removes the rAF export from ReactDOMFrameScheduling, since it's not
being used.

src/renderers/shared/ReactDOMFrameScheduling.js

@@ -25,16 +25,10 @@ import type {Deadline} from 'ReactFiberReconciler';
 var invariant = require('fbjs/lib/invariant');
 var ExecutionEnvironment = require('fbjs/lib/ExecutionEnvironment');
 
-// TODO: There's no way to cancel these, because Fiber doesn't atm.
-let rAF: (callback: (time: number) => void) => number;
+// TODO: There's no way to cancel, because Fiber doesn't atm.
 let rIC: (callback: (deadline: Deadline) => void) => number;
 
 if (!ExecutionEnvironment.canUseDOM) {
-  rAF = function(frameCallback: (time: number) => void): number {
-    setTimeout(frameCallback, 16);
-    return 0;
-  };
-
   rIC = function(frameCallback: (deadline: Deadline) => void): number {
     setTimeout(() => {
       frameCallback({
@@ -52,7 +46,7 @@ if (!ExecutionEnvironment.canUseDOM) {
       'polyfill in older browsers.',
   );
 } else if (typeof requestIdleCallback !== 'function') {
-  // Wrap requestAnimationFrame and polyfill requestIdleCallback.
+  // Polyfill requestIdleCallback.
 
   var scheduledRAFCallback = null;
   var scheduledRICCallback = null;
@@ -90,7 +84,7 @@ if (!ExecutionEnvironment.canUseDOM) {
     isIdleScheduled = false;
     var callback = scheduledRICCallback;
     scheduledRICCallback = null;
-    if (callback) {
+    if (callback !== null) {
       callback(frameDeadlineObject);
     }
   };
@@ -130,23 +124,11 @@ if (!ExecutionEnvironment.canUseDOM) {
     }
     var callback = scheduledRAFCallback;
     scheduledRAFCallback = null;
-    if (callback) {
+    if (callback !== null) {
       callback(rafTime);
     }
   };
 
-  rAF = function(callback: (time: number) => void): number {
-    // This assumes that we only schedule one callback at a time because that's
-    // how Fiber uses it.
-    scheduledRAFCallback = callback;
-    if (!isAnimationFrameScheduled) {
-      // If rIC didn't already schedule one, we need to schedule a frame.
-      isAnimationFrameScheduled = true;
-      requestAnimationFrame(animationTick);
-    }
-    return 0;
-  };
-
   rIC = function(callback: (deadline: Deadline) => void): number {
     // This assumes that we only schedule one callback at a time because that's
     // how Fiber uses it.
@@ -162,9 +144,7 @@ if (!ExecutionEnvironment.canUseDOM) {
     return 0;
   };
 } else {
-  rAF = requestAnimationFrame;
   rIC = requestIdleCallback;
 }
 
-exports.rAF = rAF;
 exports.rIC = rIC;