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/printing/constrained_window_views_client.cc

@@ -0,0 +1,42 @@
+// Copyright 2014 The Chromium 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/printing/constrained_window_views_client.h"
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "base/memory/ptr_util.h"
+#include "chrome/browser/platform_util.h"
+#include "chrome/browser/ui/browser_finder.h"
+#include "components/web_modal/web_contents_modal_dialog_host.h"
+
+namespace {
+
+class CefConstrainedWindowViewsClient
+    : public constrained_window::ConstrainedWindowViewsClient {
+ public:
+  CefConstrainedWindowViewsClient() {}
+  ~CefConstrainedWindowViewsClient() override {}
+
+ private:
+  // ConstrainedWindowViewsClient:
+  web_modal::ModalDialogHost* GetModalDialogHost(
+      gfx::NativeWindow parent) override {
+    NOTREACHED();
+    return nullptr;
+  }
+  gfx::NativeView GetDialogHostView(gfx::NativeWindow parent) override {
+    NOTREACHED();
+    return gfx::NativeView();
+  }
+
+  DISALLOW_COPY_AND_ASSIGN(CefConstrainedWindowViewsClient);
+};
+
+}  // namespace
+
+std::unique_ptr<constrained_window::ConstrainedWindowViewsClient>
+CreateCefConstrainedWindowViewsClient() {
+  return base::WrapUnique(new CefConstrainedWindowViewsClient);
+}