[heap] Fix MockPlatform in IncrementalMarkingUsingTasks test.

Now the mock platform delays all background tasks and forwards them
to the real platform in its destructor.

This fixes a race that happens when the background tasks calls
TestPlatform::MonotonicallyIncreasingTime() while the mock platform
is being destroyed.

BUG: v8:7494
Change-Id: I659ccc19121144152f447d59ff3c5e7ef1bec6d5
Reviewed-on: https://chromium-review.googlesource.com/949202Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51741}

test/cctest/heap/test-incremental-marking.cc

@@ -40,12 +40,20 @@ class MockPlatform : public TestPlatform {
   virtual ~MockPlatform() {
     delete task_;
     i::V8::SetPlatformForTesting(old_platform_);
+    for (Task* task : worker_tasks_) {
+      old_platform_->CallOnWorkerThread(task);
+    }
+    worker_tasks_.clear();
   }
 
   void CallOnForegroundThread(v8::Isolate* isolate, Task* task) override {
     task_ = task;
   }
 
+  void CallOnWorkerThread(Task* task) override {
+    worker_tasks_.push_back(task);
+  }
+
   bool IdleTasksEnabled(v8::Isolate* isolate) override { return false; }
 
   bool PendingTask() { return task_ != nullptr; }
@@ -59,6 +67,7 @@ class MockPlatform : public TestPlatform {
 
  private:
   Task* task_;
+  std::vector<Task*> worker_tasks_;
   v8::Platform* old_platform_;
 };