Remove rAF fork

**what is the change?:**
Undid facebook#12837

**why make this change?:**
We originally forked rAF because we needed to pull in a particular
version of rAF internally at Facebook, to avoid grabbing the default
polyfilled version.

The longer term solution, until we can get rid of the global polyfill
behavior, is to initialize 'schedule' before the polyfilling happens.

Now that we have landed and synced
facebook#12900 successfully, we can
initialize 'schedule' before the polyfill runs.
So we can remove the rAF fork. Here is how it will work:

1. Land this PR on Github.
2. Flarnie will quickly run a sync getting this change into www.
3. We delete the internal forked version of
   'requestAnimationFrameForReact'.
4. We require 'schedule' in the polyfill file itself, before the
   polyfilling happens.

**test plan:**
Flarnie will manually try the above steps locally and verify that things
work.

**issue:**
Internal task T29442940

packages/react-scheduler/src/ReactScheduler.js

@@ -41,17 +41,32 @@ type CallbackConfigType = {|
 
 export type CallbackIdType = CallbackConfigType;
 
-import requestAnimationFrameForReact from 'shared/requestAnimationFrameForReact';
 import ExecutionEnvironment from 'fbjs/lib/ExecutionEnvironment';
 import invariant from 'fbjs/lib/invariant';
 import warning from 'fbjs/lib/warning';
 
+import warning from 'fbjs/lib/warning';
+
+if (__DEV__) {
+  if (
+    ExecutionEnvironment.canUseDOM &&
+    typeof requestAnimationFrame !== 'function'
+  ) {
+    warning(
+      false,
+      'schedule depends on requestAnimationFrame. Make sure that you load a ' +
+        'polyfill in older browsers. https://fb.me/react-polyfills',
+    );
+  }
+}
+
 // We capture a local reference to any global, in case it gets polyfilled after
 // this module is initially evaluated.
 // We want to be using a consistent implementation.
 const localDate = Date;
 const localSetTimeout = setTimeout;
 const localClearTimeout = clearTimeout;
+const localRequestAnimationFrame = requestAnimationFrame;
 
 const hasNativePerformanceNow =
   typeof performance === 'object' && typeof performance.now === 'function';

scripts/rollup/forks.js

@@ -81,18 +81,6 @@ const forks = Object.freeze({
     return null;
   },
 
-  // This logic is forked on www to use the 'acrossTransitions' version.
-  // This will be removed soon, see internal task T29442940
-  'shared/requestAnimationFrameForReact': (bundleType, entry) => {
-    switch (bundleType) {
-      case FB_WWW_DEV:
-      case FB_WWW_PROD:
-        return 'shared/forks/requestAnimationFrameForReact.www.js';
-      default:
-        return null;
-    }
-  },
-
   'shared/ReactScheduler': (bundleType, entry) => {
     switch (bundleType) {
       case FB_WWW_DEV: