Add initial chrome runtime support (see issue #2969)

Running `cefsimple --enable-chrome-runtime` will create and run a
Chrome browser window using the CEF app methods, and call
CefApp::OnContextInitialized as expected. CEF task methods also
work as expected in the main process. No browser-related methods or
callbacks are currently supported for the Chrome window, and the
application will exit when the last Chrome window closes.

The Chrome runtime requires resources.pak, chrome_100_percent.pak
and chrome_200_percent.pak files which were not previously built
with CEF. It shares the existing locales pak files which have been
updated to include additional Chrome-specific strings.

On Linux, the Chrome runtime requires GTK so use_gtk=true must be
specified via GN_DEFINES when building.

This change also refactors the CEF runtime, which can be tested in
the various supported modes by running:
$ cefclient
$ cefclient --multi-threaded-message-loop
$ cefclient --external-message-pump

libcef/common/main_runner_handler.h

@@ -0,0 +1,24 @@
+// Copyright 2020 The Chromium Embedded Framework 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_COMMON_MAIN_RUNNER_HANDLER_H_
+#define CEF_LIBCEF_COMMON_MAIN_RUNNER_HANDLER_H_
+#pragma once
+
+namespace content {
+struct MainFunctionParams;
+}
+
+// Handles running of the main process.
+class CefMainRunnerHandler {
+ public:
+  virtual void PreCreateMainMessageLoop() = 0;
+  virtual int RunMainProcess(
+      const content::MainFunctionParams& main_function_params) = 0;
+
+ protected:
+  virtual ~CefMainRunnerHandler() {}
+};
+
+#endif  // CEF_LIBCEF_COMMON_MAIN_RUNNER_HANDLER_H_