Dedup conditional in ReactScheduler (facebook#12680)

**what is the change?:**
We had a condition to set either 'performance.now' or 'Date.now' as the
'now' function.

Then later we had another conditional checking again if
'performance.now' was supported, and using it if so, otherwise falling
back to 'Date.now'.

More efficient to just use the 'now' shortcut defined above.

**why make this change?:**
Fewer lines, clearer code.

**test plan:**
Now that we have tests we can run them :)

packages/react-scheduler/src/ReactScheduler.js

@@ -100,27 +100,13 @@ if (!ExecutionEnvironment.canUseDOM) {
   let previousFrameTime = 33;
   let activeFrameTime = 33;
 
-  let frameDeadlineObject;
-  if (hasNativePerformanceNow) {
-    frameDeadlineObject = {
-      didTimeout: false,
-      timeRemaining() {
-        // We assume that if we have a performance timer that the rAF callback
-        // gets a performance timer value. Not sure if this is always true.
-        const remaining = frameDeadline - performance.now();
-        return remaining > 0 ? remaining : 0;
-      },
-    };
-  } else {
-    frameDeadlineObject = {
-      didTimeout: false,
-      timeRemaining() {
-        // Fallback to Date.now()
-        const remaining = frameDeadline - Date.now();
-        return remaining > 0 ? remaining : 0;
-      },
-    };
-  }
+  const frameDeadlineObject = {
+    didTimeout: false,
+    timeRemaining() {
+      const remaining = frameDeadline - now();
+      return remaining > 0 ? remaining : 0;
+    },
+  };
 
   // We use the postMessage trick to defer idle work until after the repaint.
   const messageKey =