[Jobs API]: Expose missing pieces in Job to accommodate worklist users.

Expose:
- JobHandle::IsCompleted()
- worker_count passed as argument to GetMaxConcurrency()
- JobDelegate::GetTaskId
With default implementation. Once gin implements it, it can be made pure
virtual.

Bug: chromium:1114823
Change-Id: I24ce60d1df6adff4061c050e5aa8bf8d7bb1cf5a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2352485
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69388}

include/v8-platform.h

@@ -170,6 +170,14 @@ class JobDelegate {
    * details.
    */
   virtual void NotifyConcurrencyIncrease() = 0;
+
+  /**
+   * Returns a task_id unique among threads currently running this job, such
+   * that GetTaskId() < worker count. To achieve this, the same task_id may be
+   * reused by a different thread after a worker_task returns.
+   * TODO(etiennep): Make pure virtual once custom embedders implement it.
+   */
+  virtual uint8_t GetTaskId() { return 0; }
 };
 
 /**
@@ -202,6 +210,12 @@ class JobHandle {
    */
   virtual void Cancel() = 0;
 
+  /**
+   * Returns true if there's no work pending and no worker running.
+   * TODO(etiennep): Make pure virtual once custom embedders implement it.
+   */
+  virtual bool IsCompleted() { return true; }
+
   /**
    * Returns true if associated with a Job and other methods may be called.
    * Returns false after Join() or Cancel() was called.
@@ -225,6 +239,17 @@ class JobTask {
    * must not call back any JobHandle methods.
    */
   virtual size_t GetMaxConcurrency() const = 0;
+
+  /*
+   * Meant to replace the version above, given the number of threads currently
+   * assigned to this job and executing Run(). This is useful when the result
+   * must include local work items not visible globaly by other workers.
+   * TODO(etiennep): Replace the version above by this once custom embedders are
+   * migrated.
+   */
+  size_t GetMaxConcurrency(size_t worker_count) const {
+    return GetMaxConcurrency();
+  }
 };
 
 /**