Added some thread asserts to the code and made ios_surface_ safe since (#12775)

* Added some thread asserts to the code and made ios_surface_ safe since
its being written and read from different threads.

* responded to chinmays feedback, added comment

shell/platform/darwin/ios/platform_view_ios.h

@@ -49,7 +49,10 @@ class PlatformViewIOS final : public PlatformView {
 
  private:
   fml::WeakPtr<FlutterViewController> owner_controller_;
-  std::unique_ptr<IOSSurface> ios_surface_;
+  // Since the `ios_surface_` is created on the platform thread but
+  // used on the GPU thread we need to protect it with a mutex.
+  std::mutex ios_surface_mutex_;
+  std::unique_ptr<IOSSurface> ios_surface_ FML_GUARDED_BY(ios_surface_mutex_);
   std::shared_ptr<IOSGLContext> gl_context_;
   PlatformMessageRouter platform_message_router_;
   std::unique_ptr<AccessibilityBridge> accessibility_bridge_;

shell/platform/darwin/ios/platform_view_ios.mm

@@ -43,6 +43,8 @@ fml::WeakPtr<FlutterViewController> PlatformViewIOS::GetOwnerViewController() co
 }
 
 void PlatformViewIOS::SetOwnerViewController(fml::WeakPtr<FlutterViewController> owner_controller) {
+  FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
+  std::lock_guard<std::mutex> guard(ios_surface_mutex_);
   if (ios_surface_ || !owner_controller) {
     NotifyDestroyed();
     ios_surface_.reset();
@@ -92,6 +94,8 @@ void PlatformViewIOS::RegisterExternalTexture(int64_t texture_id,
 
 // |PlatformView|
 std::unique_ptr<Surface> PlatformViewIOS::CreateRenderingSurface() {
+  FML_DCHECK(task_runners_.GetGPUTaskRunner()->RunsTasksOnCurrentThread());
+  std::lock_guard<std::mutex> guard(ios_surface_mutex_);
   if (!ios_surface_) {
     FML_DLOG(INFO) << "Could not CreateRenderingSurface, this PlatformViewIOS "
                       "has no ViewController.";
@@ -102,6 +106,7 @@ std::unique_ptr<Surface> PlatformViewIOS::CreateRenderingSurface() {
 
 // |PlatformView|
 sk_sp<GrContext> PlatformViewIOS::CreateResourceContext() const {
+  FML_DCHECK(task_runners_.GetIOTaskRunner()->RunsTasksOnCurrentThread());
   if (!gl_context_ || !gl_context_->ResourceMakeCurrent()) {
     FML_DLOG(INFO) << "Could not make resource context current on IO thread. "
                       "Async texture uploads will be disabled. On Simulators, "