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.cc

@@ -0,0 +1,72 @@
+// 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.
+
+#include "libcef/browser/web_contents_dialog_helper.h"
+
+#include "libcef/browser/browser_platform_delegate.h"
+
+#include "chrome/browser/platform_util.h"
+#include "components/web_modal/web_contents_modal_dialog_manager.h"
+
+#if defined(USE_AURA)
+#include "ui/views/widget/widget.h"
+#endif
+
+CefWebContentsDialogHelper::CefWebContentsDialogHelper(
+    content::WebContents* web_contents,
+    CefBrowserPlatformDelegate* browser_delegate)
+    : browser_delegate_(browser_delegate), weak_factory_(this) {
+  web_modal::WebContentsModalDialogManager::CreateForWebContents(web_contents);
+  web_modal::WebContentsModalDialogManager::FromWebContents(web_contents)
+      ->SetDelegate(this);
+}
+
+base::RepeatingClosure CefWebContentsDialogHelper::GetBoundsChangedCallback() {
+  return base::BindRepeating(&CefWebContentsDialogHelper::OnBoundsChanged,
+                             weak_factory_.GetWeakPtr());
+}
+
+bool CefWebContentsDialogHelper::IsWebContentsVisible(
+    content::WebContents* web_contents) {
+  return platform_util::IsVisible(web_contents->GetNativeView());
+}
+
+web_modal::WebContentsModalDialogHost*
+CefWebContentsDialogHelper::GetWebContentsModalDialogHost() {
+  return this;
+}
+
+gfx::NativeView CefWebContentsDialogHelper::GetHostView() const {
+#if defined(USE_AURA)
+  return browser_delegate_->GetWindowWidget()->GetNativeView();
+#else
+  NOTIMPLEMENTED();
+  return gfx::NativeView();
+#endif
+}
+
+gfx::Point CefWebContentsDialogHelper::GetDialogPosition(
+    const gfx::Size& size) {
+  return browser_delegate_->GetDialogPosition(size);
+}
+
+gfx::Size CefWebContentsDialogHelper::GetMaximumDialogSize() {
+  return browser_delegate_->GetMaximumDialogSize();
+}
+
+void CefWebContentsDialogHelper::AddObserver(
+    web_modal::ModalDialogHostObserver* observer) {
+  if (observer && !observer_list_.HasObserver(observer))
+    observer_list_.AddObserver(observer);
+}
+
+void CefWebContentsDialogHelper::RemoveObserver(
+    web_modal::ModalDialogHostObserver* observer) {
+  observer_list_.RemoveObserver(observer);
+}
+
+void CefWebContentsDialogHelper::OnBoundsChanged() {
+  for (auto& observer : observer_list_)
+    observer.OnPositionRequiresUpdate();
+}