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/task_runner_manager.cc

@@ -0,0 +1,28 @@
+// 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.
+
+#include "libcef/common/task_runner_manager.h"
+
+#include "base/logging.h"
+
+namespace {
+
+CefTaskRunnerManager* g_manager = nullptr;
+
+}  // namespace
+
+// static
+CefTaskRunnerManager* CefTaskRunnerManager::Get() {
+  return g_manager;
+}
+
+CefTaskRunnerManager::CefTaskRunnerManager() {
+  // Only a single instance should exist.
+  DCHECK(!g_manager);
+  g_manager = this;
+}
+
+CefTaskRunnerManager::~CefTaskRunnerManager() {
+  g_manager = nullptr;
+}