Fix devtools unable to start a shared workers.

components/dom_distiller/content/browser/dom_distiller_viewer_source.cc

 // 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 "components/dom_distiller/content/browser/dom_distiller_viewer_source.h"
 
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
@@ skipping 213 matching lines @@
 
 std::string DomDistillerViewerSource::GetSource() const {
   return scheme_ + "://";
 }
 
 void DomDistillerViewerSource::StartDataRequest(
     const std::string& path,
     const content::ResourceRequestInfo::WebContentsGetter& wc_getter,
     const content::URLDataSource::GotDataCallback& callback) {
   content::WebContents* web_contents = wc_getter.Run();
+  if (!web_contents)
+    return;
   if (kViewerCssPath == path) {
     std::string css = viewer::GetCss();
     callback.Run(base::RefCountedString::TakeString(&css));
     return;
   } else if (kViewerLoadingImagePath == path) {
     std::string image = viewer::GetLoadingImage();
     callback.Run(base::RefCountedString::TakeString(&image));
     return;
   } else if (base::StartsWith(path, kViewerSaveFontScalingPath,
                               base::CompareCase::SENSITIVE)) {
@@ skipping 60 matching lines @@
 std::string DomDistillerViewerSource::GetContentSecurityPolicyStyleSrc()
     const {
   return "style-src 'self' https://fonts.googleapis.com;";
 }
 
 std::string DomDistillerViewerSource::GetContentSecurityPolicyChildSrc() const {
   return "child-src *;";
 }
 
 }  // namespace dom_distiller