Fix devtools unable to start a shared workers.

components/dom_distiller/content/browser/dom_distiller_viewer_source.cc

Index: components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
diff --git a/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc b/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
index 0058d6881072d43b55129c03dca9a7cd4c984690..e28e11d50eaa6070278cd4f2d88a02681dac9f15 100644
--- a/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
+++ b/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
@@ -231,6 +231,8 @@ void DomDistillerViewerSource::StartDataRequest(
     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));