[engine] remove merge thread setting and fix default value.

common/settings.h

@@ -232,10 +232,6 @@ struct Settings {
   // Force disable the android surface control even where supported.
   bool disable_surface_control = false;
 
-  // If true, the UI thread is the platform thread on supported
-  // platforms.
-  bool merged_platform_ui_thread = true;
-
   // Log a warning during shell initialization if Impeller is not enabled.
   bool warn_on_impeller_opt_out = false;
 

shell/common/switches.cc

@@ -360,9 +360,6 @@ Settings SettingsFromCommandLine(const fml::CommandLine& command_line) {
   settings.verbose_logging =
       command_line.HasOption(FlagForSwitch(Switch::VerboseLogging));
 
-  settings.merged_platform_ui_thread = command_line.HasOption(
-      FlagForSwitch(Switch::EnableMergedPlatformUIThread));
-
   command_line.GetOptionValue(FlagForSwitch(Switch::FlutterAssetsDir),
                               &settings.assets_path);
 

shell/platform/android/android_shell_holder.cc

@@ -90,9 +90,6 @@ AndroidShellHolder::AndroidShellHolder(
   auto thread_label = std::to_string(thread_host_count++);
 
   auto mask = ThreadHost::Type::kRaster | ThreadHost::Type::kIo;
-  if (!settings.merged_platform_ui_thread) {
-    mask |= ThreadHost::Type::kUi;
-  }
 
   flutter::ThreadHost::ThreadHostConfig host_config(
       thread_label, mask, AndroidPlatformThreadConfigSetter);
@@ -139,11 +136,7 @@ AndroidShellHolder::AndroidShellHolder(
   fml::RefPtr<fml::TaskRunner> platform_runner =
       fml::MessageLoop::GetCurrent().GetTaskRunner();
   raster_runner = thread_host_->raster_thread->GetTaskRunner();
-  if (settings.merged_platform_ui_thread) {
-    ui_runner = platform_runner;
-  } else {
-    ui_runner = thread_host_->ui_thread->GetTaskRunner();
-  }
+  ui_runner = platform_runner;
   io_runner = thread_host_->io_thread->GetTaskRunner();
 
   flutter::TaskRunners task_runners(thread_label,     // label

shell/platform/android/android_shell_holder_unittests.cc

@@ -163,7 +163,6 @@ TEST(AndroidShellHolder, HandlePlatformMessage) {
 
 TEST(AndroidShellHolder, CreateWithMergedPlatformAndUIThread) {
   Settings settings;
-  settings.merged_platform_ui_thread = true;
   auto jni = std::make_shared<MockPlatformViewAndroidJNI>();
   auto holder = std::make_unique<AndroidShellHolder>(settings, jni);
   auto window = fml::MakeRefCounted<AndroidNativeWindow>(
@@ -175,19 +174,5 @@ TEST(AndroidShellHolder, CreateWithMergedPlatformAndUIThread) {
       holder->GetShellForTesting()->GetTaskRunners().GetPlatformTaskRunner());
 }
 
-TEST(AndroidShellHolder, CreateWithUnMergedPlatformAndUIThread) {
-  Settings settings;
-  settings.merged_platform_ui_thread = false;
-  auto jni = std::make_shared<MockPlatformViewAndroidJNI>();
-  auto holder = std::make_unique<AndroidShellHolder>(settings, jni);
-  auto window = fml::MakeRefCounted<AndroidNativeWindow>(
-      nullptr, /*is_fake_window=*/true);
-  holder->GetPlatformView()->NotifyCreated(window);
-
-  EXPECT_NE(
-      holder->GetShellForTesting()->GetTaskRunners().GetUITaskRunner(),
-      holder->GetShellForTesting()->GetTaskRunners().GetPlatformTaskRunner());
-}
-
 }  // namespace testing
 }  // namespace flutter