cppgc: Sweeper: Get task runner only when scheduling sweeping

Platform::GetForegroundTaskRunner() can only be used after attaching
an Isolate in V8. Work around that problem by getting the runner only
when needed.

Bug: chromium:1056170
Change-Id: If15ec691e7f5cf11be8b7a3bc18827246ac083d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2674009
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72527}

src/heap/cppgc/sweeper.cc

@@ -526,8 +526,7 @@ class Sweeper::SweeperImpl final {
       : heap_(heap),
         stats_collector_(stats_collector),
         space_states_(heap->size()),
-        platform_(platform),
-        foreground_task_runner_(platform_->GetForegroundTaskRunner()) {}
+        platform_(platform) {}
 
   ~SweeperImpl() { CancelSweepers(); }
 
@@ -730,12 +729,11 @@ class Sweeper::SweeperImpl final {
 
   void ScheduleIncrementalSweeping() {
     DCHECK(platform_);
-    if (!foreground_task_runner_ ||
-        !foreground_task_runner_->IdleTasksEnabled())
-      return;
+    auto runner = platform_->GetForegroundTaskRunner();
+    if (!runner || !runner->IdleTasksEnabled()) return;
 
     incremental_sweeper_handle_ =
-        IncrementalSweepTask::Post(this, foreground_task_runner_.get());
+        IncrementalSweepTask::Post(this, runner.get());
   }
 
   void ScheduleConcurrentSweeping() {
@@ -763,7 +761,6 @@ class Sweeper::SweeperImpl final {
   StatsCollector* stats_collector_;
   SpaceStates space_states_;
   cppgc::Platform* platform_;
-  std::shared_ptr<cppgc::TaskRunner> foreground_task_runner_;
   IncrementalSweepTask::Handle incremental_sweeper_handle_;
   std::unique_ptr<cppgc::JobHandle> concurrent_sweeper_handle_;
   // Indicates whether the sweeping phase is in progress.