Add back lastPendingTime field

This reverts commit 9a54113.

I want to use this so we can check if there might be any lower priority
updates in a suspended tree.

We can remove it again during the expiration times refactor.

packages/react-reconciler/src/ReactFiberRoot.js

@@ -65,6 +65,8 @@ type BaseFiberRootProperties = {|
   callbackPriority: ReactPriorityLevel,
   // The earliest pending expiration time that exists in the tree
   firstPendingTime: ExpirationTime,
+  // The latest pending expiration time that exists in the tree
+  lastPendingTime: ExpirationTime,
   // The earliest suspended expiration time that exists in the tree
   firstSuspendedTime: ExpirationTime,
   // The latest suspended expiration time that exists in the tree
@@ -122,6 +124,7 @@ function FiberRootNode(containerInfo, tag, hydrate) {
   this.callbackNode = null;
   this.callbackPriority = NoPriority;
   this.firstPendingTime = NoWork;
+  this.lastPendingTime = NoWork;
   this.firstSuspendedTime = NoWork;
   this.lastSuspendedTime = NoWork;
   this.nextKnownPendingLevel = NoWork;
@@ -205,6 +208,10 @@ export function markRootUpdatedAtTime(
   if (expirationTime > firstPendingTime) {
     root.firstPendingTime = expirationTime;
   }
+  const lastPendingTime = root.lastPendingTime;
+  if (lastPendingTime === NoWork || expirationTime < lastPendingTime) {
+    root.lastPendingTime = expirationTime;
+  }
 
   // Update the range of suspended times. Treat everything lower priority or
   // equal to this update as unsuspended.
@@ -232,6 +239,11 @@ export function markRootFinishedAtTime(
 ): void {
   // Update the range of pending times
   root.firstPendingTime = remainingExpirationTime;
+  if (remainingExpirationTime < root.lastPendingTime) {
+    // This usually means we've finished all the work, but it can also happen
+    // when something gets downprioritized during render, like a hidden tree.
+    root.lastPendingTime = remainingExpirationTime;
+  }
 
   // Update the range of suspended times. Treat everything higher priority or
   // equal to this update as unsuspended.