Remove main runner abstractions (see #3685)

Remove code abstractions that are no longer required after deletion of
the Alloy bootstrap. This is a functional no-op.

libcef/browser/ui_thread.h

@@ -0,0 +1,63 @@
+// Copyright 2020 The Chromium Embedded Framework Authors.
+// Portions copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CEF_LIBCEF_BROWSER_UI_THREAD_H_
+#define CEF_LIBCEF_BROWSER_UI_THREAD_H_
+#pragma once
+
+#include <memory>
+
+#include "base/functional/callback_forward.h"
+#include "base/memory/raw_ptr.h"
+#include "base/sequence_checker.h"
+#include "base/synchronization/lock.h"
+#include "base/synchronization/waitable_event.h"
+#include "base/threading/platform_thread.h"
+#include "content/public/browser/browser_main_runner.h"
+#include "content/public/common/main_function_params.h"
+
+class CefMainRunner;
+
+// Used to run the UI on a separate thread.
+class CefUIThread final : public base::PlatformThread::Delegate {
+ public:
+  CefUIThread(CefMainRunner* runner, base::OnceClosure setup_callback);
+  ~CefUIThread() override;
+
+  void Start();
+  void Stop();
+
+  bool WaitUntilThreadStarted() const;
+
+  void InitializeBrowserRunner(
+      content::MainFunctionParams main_function_params);
+
+  void set_shutdown_callback(base::OnceClosure shutdown_callback) {
+    shutdown_callback_ = std::move(shutdown_callback);
+  }
+
+ private:
+  void ThreadMain() override;
+
+  const raw_ptr<CefMainRunner> runner_;
+  base::OnceClosure setup_callback_;
+  base::OnceClosure shutdown_callback_;
+
+  std::unique_ptr<content::BrowserMainRunner> browser_runner_;
+
+  bool stopping_ = false;
+
+  // The thread's handle.
+  base::PlatformThreadHandle thread_;
+  mutable base::Lock thread_lock_;  // Protects |thread_|.
+
+  mutable base::WaitableEvent start_event_;
+
+  // This class is not thread-safe, use this to verify access from the owning
+  // sequence of the Thread.
+  base::SequenceChecker owning_sequence_checker_;
+};
+
+#endif  // CEF_LIBCEF_BROWSER_UI_THREAD_H_