[include] Make Platform::CreateJob abstract

Embedders must override the method, because returning a nullptr will
make V8 crash. Hence the method should be abstract.

Bug: v8:12425
Change-Id: I79e1759acd2a5f41424145637ee1fbd161889ec1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3779694Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82058}

include/v8-platform.h

@@ -1071,13 +1071,9 @@ class Platform {
    *    return v8::platform::NewDefaultJobHandle(
    *        this, priority, std::move(job_task), NumberOfWorkerThreads());
    * }
-   *
-   * TODO(etiennep): Make pure virtual once custom embedders implement it.
    */
   virtual std::unique_ptr<JobHandle> CreateJob(
-      TaskPriority priority, std::unique_ptr<JobTask> job_task) {
-    return nullptr;
-  }
+      TaskPriority priority, std::unique_ptr<JobTask> job_task) = 0;
 
   /**
    * Monotonically increasing time in seconds from an arbitrary fixed point in