[platform] Provide NewDefaultJobHandle function

This will allow embedders to actually instantiate the {DefaultJobHandle}
as suggested in the comment. Node currently implements {PostJob()} by
just returning a nullptr. After this change, it can use the new
{NewDefaultJobHandle} function and we can actually start using this API
in v8.

R=adamk@chromium.org

Bug: v8:10723
Change-Id: I4b31a640d0edc7e7207d1df95e683465dfaaaeff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2304812Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68934}

include/libplatform/libplatform.h

@@ -43,6 +43,16 @@ V8_PLATFORM_EXPORT std::unique_ptr<v8::Platform> NewDefaultPlatform(
         InProcessStackDumping::kDisabled,
     std::unique_ptr<v8::TracingController> tracing_controller = {});
 
+/**
+ * Returns a new instance of the default v8::JobHandle implementation.
+ *
+ * The job will be executed by spawning up to |num_worker_threads| many worker
+ * threads on the provided |platform| with the given |priority|.
+ */
+V8_PLATFORM_EXPORT std::unique_ptr<v8::JobHandle> NewDefaultJobHandle(
+    v8::Platform* platform, v8::TaskPriority priority,
+    std::unique_ptr<v8::JobTask> job_task, size_t num_worker_threads);
+
 /**
  * Pumps the message loop for the given isolate.
  *

include/v8-platform.h

@@ -535,9 +535,8 @@ class Platform {
    * libplatform looks like:
    *  std::unique_ptr<JobHandle> PostJob(
    *      TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
-   *    return std::make_unique<DefaultJobHandle>(
-   *        std::make_shared<DefaultJobState>(
-   *            this, std::move(job_task), kNumThreads));
+   *    return v8::platform::NewDefaultJobHandle(
+   *        this, priority, std::move(job_task), NumberOfWorkerThreads());
    * }
    */
   virtual std::unique_ptr<JobHandle> PostJob(

src/libplatform/default-platform.cc

@@ -45,6 +45,13 @@ std::unique_ptr<v8::Platform> NewDefaultPlatform(
   return platform;
 }
 
+V8_PLATFORM_EXPORT std::unique_ptr<JobHandle> NewDefaultJobHandle(
+    Platform* platform, TaskPriority priority,
+    std::unique_ptr<JobTask> job_task, size_t num_worker_threads) {
+  return std::make_unique<DefaultJobHandle>(std::make_shared<DefaultJobState>(
+      platform, std::move(job_task), priority, num_worker_threads));
+}
+
 bool PumpMessageLoop(v8::Platform* platform, v8::Isolate* isolate,
                      MessageLoopBehavior behavior) {
   return static_cast<DefaultPlatform*>(platform)->PumpMessageLoop(isolate,