Revert "Replace array<fml::Thread, 3> with ThreadHost. (#27455)"

This reverts commit 9e5999d7ca9490060a23d0521c130be3f89fdb25.

shell/platform/fuchsia/flutter/engine.cc

diff --git a/shell/platform/fuchsia/flutter/engine.cc b/shell/platform/fuchsia/flutter/engine.cc
index 350e0e4..0c5c615 100644
--- a/shell/platform/fuchsia/flutter/engine.cc
+++ b/shell/platform/fuchsia/flutter/engine.cc
@@ -34,6 +34,23 @@
 namespace flutter_runner {
 namespace {
 
+void UpdateNativeThreadLabelNames(const std::string& label,
+                                  const flutter::TaskRunners& runners) {
+  auto set_thread_name = [](fml::RefPtr<fml::TaskRunner> runner,
+                            std::string prefix, std::string suffix) {
+    if (!runner) {
+      return;
+    }
+    fml::TaskRunner::RunNowOrPostTask(runner, [name = prefix + suffix]() {
+      zx::thread::self()->set_property(ZX_PROP_NAME, name.c_str(), name.size());
+    });
+  };
+  set_thread_name(runners.GetPlatformTaskRunner(), label, ".platform");
+  set_thread_name(runners.GetUITaskRunner(), label, ".ui");
+  set_thread_name(runners.GetRasterTaskRunner(), label, ".raster");
+  set_thread_name(runners.GetIOTaskRunner(), label, ".io");
+}
+
 std::unique_ptr<flutter::PlatformMessage> MakeLocalizationPlatformMessage(
     const fuchsia::intl::Profile& intl_profile) {
   return std::make_unique<flutter::PlatformMessage>(
@@ -55,10 +72,6 @@
                FlutterRunnerProductConfiguration product_config)
     : delegate_(delegate),
       thread_label_(std::move(thread_label)),
-      thread_host_(thread_label_ + ".",
-                   flutter::ThreadHost::Type::RASTER |
-                       flutter::ThreadHost::Type::UI |
-                       flutter::ThreadHost::Type::IO),
       intercept_all_input_(product_config.get_intercept_all_input()),
       weak_factory_(this) {
   // Get the task runners from the managed threads. The current thread will be
@@ -67,12 +80,13 @@
       fml::MessageLoop::GetCurrent().GetTaskRunner();
 
   const flutter::TaskRunners task_runners(
-      thread_label_,                                // Dart thread labels
-      platform_task_runner,                         // platform
-      thread_host_.raster_thread->GetTaskRunner(),  // raster
-      thread_host_.ui_thread->GetTaskRunner(),      // ui
-      thread_host_.io_thread->GetTaskRunner()       // io
+      thread_label_,                // Dart thread labels
+      platform_task_runner,         // platform
+      threads_[0].GetTaskRunner(),  // raster
+      threads_[1].GetTaskRunner(),  // ui
+      threads_[2].GetTaskRunner()   // io
   );
+  UpdateNativeThreadLabelNames(thread_label_, task_runners);
 
   // Connect to Scenic.
   auto scenic = svc->Connect<fuchsia::ui::scenic::Scenic>();
@@ -470,6 +484,9 @@
 
 Engine::~Engine() {
   shell_.reset();
+  for (auto& thread : threads_) {
+    thread.Join();
+  }
 }
 
 std::optional<uint32_t> Engine::GetEngineReturnCode() const {

shell/platform/fuchsia/flutter/engine.h

diff --git a/shell/platform/fuchsia/flutter/engine.h b/shell/platform/fuchsia/flutter/engine.h
index bc7a66f..33c704f 100644
--- a/shell/platform/fuchsia/flutter/engine.h
+++ b/shell/platform/fuchsia/flutter/engine.h
@@ -21,7 +21,6 @@
 #include "flutter/flow/surface.h"
 #include "flutter/fml/macros.h"
 #include "flutter/shell/common/shell.h"
-#include "flutter/shell/common/thread_host.h"
 #include "flutter/shell/platform/fuchsia/flutter/accessibility_bridge.h"
 
 #include "flutter_runner_product_configuration.h"
@@ -69,7 +68,7 @@
   Delegate& delegate_;
 
   const std::string thread_label_;
-  flutter::ThreadHost thread_host_;
+  std::array<fml::Thread, 3> threads_;
 
   std::shared_ptr<GfxSessionConnection> session_connection_;
   std::optional<VulkanSurfaceProducer> surface_producer_;