[fuchsia] Bind ChildViewWatcher on platform thread (#42222)

This CL posts ChildViewWatcher calls on platform thread.

Bug: fxb/125256

Author: uysalere

shell/platform/fuchsia/flutter/flatland_platform_view.cc

@@ -182,43 +182,43 @@ void FlatlandPlatformView::OnCreateView(ViewCallback on_view_created,
     FML_CHECK(weak);
     FML_CHECK(weak->child_view_info_.count(content_id.value) == 0);
 
-    // Bind the child view watcher to the platform thread so that the FIDL calls
-    // are handled on the platform thread.
-    fuchsia::ui::composition::ChildViewWatcherPtr child_view_watcher =
-        child_view_watcher_handle.Bind();
-    FML_CHECK(child_view_watcher);
-
-    child_view_watcher.set_error_handler(
-        [weak, view_id, content_id](zx_status_t status) {
-          FML_LOG(WARNING) << "Child disconnected. ChildViewWatcher status: "
-                           << status;
-
+    platform_task_runner->PostTask(fml::MakeCopyable(
+        [weak, view_id, content_id,
+         watcher_handle = std::move(child_view_watcher_handle)]() mutable {
           if (!weak) {
             FML_LOG(WARNING)
                 << "Flatland View bound to PlatformView after PlatformView was "
                    "destroyed; ignoring.";
             return;
           }
 
-          // Disconnected views cannot listen to pointer events.
-          weak->pointer_injector_delegate_->OnDestroyView(view_id);
+          // Bind the child view watcher to the platform thread so that the FIDL
+          // calls are handled on the platform thread.
+          fuchsia::ui::composition::ChildViewWatcherPtr child_view_watcher =
+              watcher_handle.Bind();
+          FML_CHECK(child_view_watcher);
 
-          weak->OnChildViewDisconnected(content_id.value);
-        });
-
-    platform_task_runner->PostTask(
-        fml::MakeCopyable([weak, view_id, content_id,
-                           watcher = std::move(child_view_watcher)]() mutable {
-          if (!weak) {
+          child_view_watcher.set_error_handler([weak, view_id, content_id](
+                                                   zx_status_t status) {
             FML_LOG(WARNING)
-                << "Flatland View bound to PlatformView after PlatformView was "
-                   "destroyed; ignoring.";
-            return;
-          }
+                << "Child disconnected. ChildViewWatcher status: " << status;
+
+            if (!weak) {
+              FML_LOG(WARNING) << "Flatland View bound to PlatformView after "
+                                  "PlatformView was "
+                                  "destroyed; ignoring.";
+              return;
+            }
+
+            // Disconnected views cannot listen to pointer events.
+            weak->pointer_injector_delegate_->OnDestroyView(view_id);
+
+            weak->OnChildViewDisconnected(content_id.value);
+          });
 
           weak->child_view_info_.emplace(
               std::piecewise_construct, std::forward_as_tuple(content_id.value),
-              std::forward_as_tuple(view_id, std::move(watcher)));
+              std::forward_as_tuple(view_id, std::move(child_view_watcher)));
 
           weak->child_view_info_.at(content_id.value)
               .child_view_watcher->GetStatus(