[d8] Fix job execution on the predictable platform

Jobs were still being posted on the underlying default platform, which
caused concurrent execution. By directly returning a
{NewDefaultJobHandle} with a pointer to the {PredictablePlatform}, we
force execution of all posted tasks via that platform.

R=ahaas@chromium.org, cbruni@chromium.org

Bug: v8:11848
Change-Id: Ie10519583341b427776ca428f85641e96f821367
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2944808Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75002}

src/DEPS

@@ -73,6 +73,9 @@ specific_include_rules = {
     "+include/libplatform/v8-tracing.h",
     "+perfetto/tracing.h"
   ],
+  "d8-platforms\.cc": [
+    "+include/libplatform/libplatform.h",
+  ],
   "builtins-trace\.cc": [
     "+protos/perfetto",
   ],

src/d8/d8-platforms.cc

@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "src/d8/d8-platforms.h"
+
 #include <memory>
 #include <unordered_map>
 
+#include "include/libplatform/libplatform.h"
 #include "include/v8-platform.h"
 #include "src/base/logging.h"
 #include "src/base/macros.h"
@@ -12,7 +15,6 @@
 #include "src/base/platform/platform.h"
 #include "src/base/platform/time.h"
 #include "src/base/utils/random-number-generator.h"
-#include "src/d8/d8-platforms.h"
 
 namespace v8 {
 
@@ -43,7 +45,12 @@ class PredictablePlatform final : public Platform {
     return platform_->GetForegroundTaskRunner(isolate);
   }
 
-  int NumberOfWorkerThreads() override { return 0; }
+  int NumberOfWorkerThreads() override {
+    // The predictable platform executes everything on the main thread, but we
+    // still pretend to have the default number of worker threads to not
+    // unnecessarily change behaviour of the platform.
+    return platform_->NumberOfWorkerThreads();
+  }
 
   void CallOnWorkerThread(std::unique_ptr<Task> task) override {
     // We post worker tasks on the foreground task runner of the
@@ -68,7 +75,10 @@ class PredictablePlatform final : public Platform {
 
   std::unique_ptr<JobHandle> PostJob(
       TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
-    return platform_->PostJob(priority, std::move(job_task));
+    // Do not call {platform_->PostJob} here, as this would create a job that
+    // posts tasks directly to the underlying default platform.
+    return platform::NewDefaultJobHandle(this, priority, std::move(job_task),
+                                         NumberOfWorkerThreads());
   }
 
   double MonotonicallyIncreasingTime() override {