[Flight] Move pendingChunks ref count increment into createTask (#28260)

Every time we create a task we need to wait for it so we increase a ref
count. We can do this in `createTask`. This is in line with what Fizz
does too.

They differ in that Flight counts when they're actually flushed where as
Fizz decrements them when they complete.

Flight should probably count them when they complete so it's possible to
wait for the end before flushing for buffering purposes.

Author: sebmarkbage

packages/react-server/src/ReactFlightServer.js

@@ -296,7 +296,6 @@ export function createRequest(
     onError: onError === undefined ? defaultErrorHandler : onError,
     onPostpone: onPostpone === undefined ? defaultPostponeHandler : onPostpone,
   };
-  request.pendingChunks++;
   const rootTask = createTask(request, model, null, false, abortSet);
   pingedTasks.push(rootTask);
   return request;
@@ -318,7 +317,6 @@ function serializeThenable(
   task: Task,
   thenable: Thenable<any>,
 ): number {
-  request.pendingChunks++;
   const newTask = createTask(
     request,
     null,
@@ -722,6 +720,7 @@ function createTask(
   implicitSlot: boolean,
   abortSet: Set<Task>,
 ): Task {
+  request.pendingChunks++;
   const id = request.nextChunkId++;
   if (typeof model === 'object' && model !== null) {
     // If we're about to write this into a new task we can assign it an ID early so that
@@ -906,7 +905,6 @@ function serializeClientReference(
 }
 
 function outlineModel(request: Request, value: ReactClientValue): number {
-  request.pendingChunks++;
   const newTask = createTask(
     request,
     value,
@@ -1068,7 +1066,6 @@ function renderModel(
       // $FlowFixMe[method-unbinding]
       if (typeof x.then === 'function') {
         // Something suspended, we'll need to create a new task and resolve it later.
-        request.pendingChunks++;
         const newTask = createTask(
           request,
           task.model,