[v8 platform] Introduce CallBlockingTaskFromBackgroundThread()

and use it for parallel GC as it blocks the main thread.

On Chromium this will result in a TaskPriority::USER_BLOCKING task,
allowing TaskScheduler to prioritize them above all other async work.

R=ahaas@chromium.org

Bug: chromium:817421
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I8c782eb045035ce2899214528deae5a45f0e6600
Reviewed-on: https://chromium-review.googlesource.com/941946
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51650}

include/v8-platform.h

@@ -346,6 +346,15 @@ class Platform {
     CallOnBackgroundThread(task, kShortRunningTask);
   }
 
+  /**
+   * Schedules a task that blocks the main thread to be invoked with
+   * high-priority on a worker thread.
+   */
+  virtual void CallBlockingTaskOnWorkerThread(Task* task) {
+    // TODO(gab): Make pure-virtual when all embedders override this.
+    CallOnWorkerThread(task);
+  }
+
   /**
    * Schedules a task to be invoked on a foreground thread wrt a specific
    * |isolate|. Tasks posted for the same isolate should be execute in order of

src/heap/item-parallel-job.cc

@@ -105,7 +105,7 @@ void ItemParallelJob::Run(std::shared_ptr<Counters> async_counters) {
                               : base::Optional<AsyncTimedHistogram>());
     task_ids[i] = task->id();
     if (i > 0) {
-      V8::GetCurrentPlatform()->CallOnWorkerThread(task);
+      V8::GetCurrentPlatform()->CallBlockingTaskOnWorkerThread(task);
     } else {
       main_task = task;
     }