Add CefAppManager and remove global ContentClient accessors (see issue #2969)

This is the first pass in removing direct dependencies on the Alloy
runtime from code that can potentially be shared between runtimes.

CefBrowserHost and CefRequestContext APIs (including CefCookieManager,
CefURLRequest, etc.) are not yet implemented for the Chrome runtime.
Assert early if these API methods are called while the Chrome runtime
is enabled.

libcef/features/runtime_checks.h

@@ -0,0 +1,18 @@
+// 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_FEATURES_RUNTIME_CHECKS_H_
+#define CEF_LIBCEF_FEATURES_RUNTIME_CHECKS_H_
+#pragma once
+
+#include "base/logging.h"
+#include "cef/libcef/features/runtime.h"
+
+#define REQUIRE_ALLOY_RUNTIME() \
+  CHECK(cef::IsAlloyRuntimeEnabled()) << "Alloy runtime is required"
+
+#define REQUIRE_CHROME_RUNTIME() \
+  CHECK(cef::IsChrimeRuntimeEnabled()) << "Chrome runtime is required"
+
+#endif  // CEF_LIBCEF_FEATURES_RUNTIME_CHECKS_H_