[platform] Implement TaskRunners on the PredictablePlatform

On the PredictablePlatform we return the ForegroundTaskRunner of the
underlying platform in both GetForeGroundTaskRunner and
GetBackgroundTaskRunner. The reason is that thereby we can enforce a
predictable, sequential execution of tasks.

R=clemensh@chromium.org, rmcilroy@chromium.org

Change-Id: Icec9fe52da922b1e75a3fb5b0155083be0a3a0fd
Reviewed-on: https://chromium-review.googlesource.com/771792Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49384}

src/d8.cc

@@ -211,6 +211,18 @@ class PredictablePlatform : public Platform {
     DCHECK_NOT_NULL(platform_);
   }
 
+  std::shared_ptr<TaskRunner> GetForegroundTaskRunner(
+      v8::Isolate* isolate) override {
+    return platform_->GetForegroundTaskRunner(isolate);
+  }
+
+  std::shared_ptr<TaskRunner> GetBackgroundTaskRunner(
+      v8::Isolate* isolate) override {
+    // Return the foreground task runner here, so that all tasks get executed
+    // sequentially in a predictable order.
+    return platform_->GetForegroundTaskRunner(isolate);
+  }
+
   void CallOnBackgroundThread(Task* task,
                               ExpectedRuntime expected_runtime) override {
     // It's not defined when background tasks are being executed, so we can just