Add support for direct DevTools protocol messaging (fixes issue #2961).

This change allows the client to directly send and receive DevTools
protocol messages (send method calls, and receive method results and
events) without requiring a DevTools front-end or remote-debugging
session.

This change includes additional supporting changes:
- Add a new CefRequestHandler::OnDocumentAvailableInMainFrame
  callback (see issue #1454).
- Add a CefParseJSON variant that accepts a UTF8-encoded buffer.
- Add a `--devtools-protocol-log-file=<path>` command-line flag for
  logging protocol messages sent to/from the DevTools front-end
  while it is displayed. This is useful for understanding existing
  DevTools protocol usage.
- Add a new "libcef_static_unittests" executable target to support
  light-weight unit tests of libcef_static internals (e.g. without
  requiring exposure via the CEF API). Files to be unittested are
  placed in the new "libcef_static_unittested" source_set which is
  then included by both the existing libcef_static library and the
  new unittests executable target.
- Linux: Remove use_bundled_fontconfig=false, which is no longer
  required and causes unittest build errors (see issue #2424).

This change also adds a cefclient demo for configuring offline mode
using the DevTools protocol (fixes issue #245). This is controlled
by the "Offline mode" context menu option and the `--offline`
command-line switch which will launch cefclient in offline mode. When
cefclient is offline all network requests will fail with
ERR_INTERNET_DISCONNECTED and navigator.onLine will return false when
called from JavaScript in any frame. This mode is per-browser so
newly created browser windows will have the default mode. Note that
configuring offline mode in this way will not update the Network tab
UI ("Throtting" option) in a displayed DevTools front-end instance.

libcef/browser/devtools/devtools_controller.h

@@ -0,0 +1,79 @@
+// Copyright (c) 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_BROWSER_DEVTOOLS_DEVTOOLS_CONTROLLER_H_
+#define CEF_LIBCEF_BROWSER_DEVTOOLS_DEVTOOLS_CONTROLLER_H_
+
+#include <memory>
+
+#include "content/public/browser/devtools_agent_host_client.h"
+
+#include "base/containers/span.h"
+#include "base/memory/scoped_refptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/observer_list.h"
+#include "base/values.h"
+
+namespace content {
+class WebContents;
+}
+
+class CefDevToolsController : public content::DevToolsAgentHostClient {
+ public:
+  class Observer : public base::CheckedObserver {
+   public:
+    // See CefDevToolsMessageObserver documentation.
+    virtual bool OnDevToolsMessage(const base::StringPiece& message) = 0;
+    virtual void OnDevToolsMethodResult(int message_id,
+                                        bool success,
+                                        const base::StringPiece& result) = 0;
+    virtual void OnDevToolsEvent(const base::StringPiece& method,
+                                 const base::StringPiece& params) = 0;
+    virtual void OnDevToolsAgentAttached() = 0;
+    virtual void OnDevToolsAgentDetached() = 0;
+
+    virtual void OnDevToolsControllerDestroyed() = 0;
+
+   protected:
+    ~Observer() override {}
+  };
+
+  // |inspected_contents| will outlive this object.
+  explicit CefDevToolsController(content::WebContents* inspected_contents);
+  ~CefDevToolsController() override;
+
+  // See CefBrowserHost methods of the same name for documentation.
+  bool SendDevToolsMessage(const base::StringPiece& message);
+  int ExecuteDevToolsMethod(int message_id,
+                            const std::string& method,
+                            const base::DictionaryValue* params);
+
+  // |observer| must outlive this object or be removed.
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+  base::WeakPtr<CefDevToolsController> GetWeakPtr() {
+    return weak_ptr_factory_.GetWeakPtr();
+  }
+
+ private:
+  // content::DevToolsAgentHostClient implementation:
+  void AgentHostClosed(content::DevToolsAgentHost* agent_host) override;
+  void DispatchProtocolMessage(content::DevToolsAgentHost* agent_host,
+                               base::span<const uint8_t> message) override;
+
+  bool EnsureAgentHost();
+
+  content::WebContents* const inspected_contents_;
+  scoped_refptr<content::DevToolsAgentHost> agent_host_;
+  int next_message_id_ = 1;
+
+  base::ObserverList<Observer> observers_;
+
+  base::WeakPtrFactory<CefDevToolsController> weak_ptr_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(CefDevToolsController);
+};
+
+#endif  // CEF_LIBCEF_BROWSER_DEVTOOLS_DEVTOOLS_CONTROLLER_H_