add print to pdf for headless

headless/lib/renderer/headless_print_web_view_helper_delegate.cc

+// Copyright (c) 2017 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 "headless/lib/renderer/headless_print_web_view_helper_delegate.h"
+
+#include "third_party/WebKit/public/web/WebElement.h"
+
+namespace headless {
+
+HeadlessPrintWebViewHelperDelegate::~HeadlessPrintWebViewHelperDelegate() {}
+
+bool HeadlessPrintWebViewHelperDelegate::CancelPrerender(
+    content::RenderFrame* render_frame) {
+  return false;
+}
+
+blink::WebElement HeadlessPrintWebViewHelperDelegate::GetPdfElement(
+    blink::WebLocalFrame* frame) {
+  return blink::WebElement();
+}
+
+bool HeadlessPrintWebViewHelperDelegate::IsPrintPreviewEnabled() {
+  return false;
+}
+
+bool HeadlessPrintWebViewHelperDelegate::OverridePrint(
+    blink::WebLocalFrame* frame) {
+  return false;
+}
+
+bool HeadlessPrintWebViewHelperDelegate::IsAskPrintSettingsEnabled() {

[Lei Zhang, 2017/03/29 01:38:03]

For later - You may want to flip this to true, so you can feed in the settings
from the user however you like. WebView did this recently in
https://codereview.chromium.org/2770713003/

[jzfeng, 2017/03/29 03:50:14]

This cl is helpful! The current plan is to let users feed the settings through
devtools command instead of using a fixed one.

+  return false;
+}
+}