Add print preview support (see issue #123)

Pass the `--enable-print-preview` command-line flag to enable. Currently only
supported on Windows and Linux.

libcef/browser/web_contents_dialog_helper.h

@@ -0,0 +1,51 @@
+// Copyright (c) 2019 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_WEB_CONTENTS_DIALOG_HELPER_H_
+#define CEF_LIBCEF_BROWSER_WEB_CONTENTS_DIALOG_HELPER_H_
+#pragma once
+
+#include "base/callback_forward.h"
+#include "base/memory/weak_ptr.h"
+#include "base/observer_list.h"
+#include "components/web_modal/modal_dialog_host.h"
+#include "components/web_modal/web_contents_modal_dialog_host.h"
+#include "components/web_modal/web_contents_modal_dialog_manager_delegate.h"
+
+class CefBrowserPlatformDelegate;
+
+class CefWebContentsDialogHelper
+    : public web_modal::WebContentsModalDialogManagerDelegate,
+      public web_modal::WebContentsModalDialogHost {
+ public:
+  CefWebContentsDialogHelper(content::WebContents* web_contents,
+                             CefBrowserPlatformDelegate* browser_delegate);
+
+  base::RepeatingClosure GetBoundsChangedCallback();
+
+  // web_modal::WebContentsModalDialogManagerDelegate methods:
+  bool IsWebContentsVisible(content::WebContents* web_contents) override;
+  web_modal::WebContentsModalDialogHost* GetWebContentsModalDialogHost()
+      override;
+
+  // web_modal::WebContentsModalDialogHost methods:
+  gfx::NativeView GetHostView() const override;
+  gfx::Point GetDialogPosition(const gfx::Size& size) override;
+  gfx::Size GetMaximumDialogSize() override;
+  void AddObserver(web_modal::ModalDialogHostObserver* observer) override;
+  void RemoveObserver(web_modal::ModalDialogHostObserver* observer) override;
+
+ private:
+  void OnBoundsChanged();
+
+  CefBrowserPlatformDelegate* const browser_delegate_;
+
+  // Used to notify WebContentsModalDialog.
+  base::ObserverList<web_modal::ModalDialogHostObserver>::Unchecked
+      observer_list_;
+
+  base::WeakPtrFactory<CefWebContentsDialogHelper> weak_factory_;
+};
+
+#endif  // CEF_LIBCEF_BROWSER_WEB_CONTENTS_DIALOG_HELPER_H_