[dom_distiller] Fix crash for invalid chrome-distiller:// requests

components/dom_distiller/content/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/dom_distiller_viewer_source.h"
 
 #include <sstream>
 #include <string>
 #include <vector>
 
@@ skipping 255 matching lines @@
   if (kViewerJsPath == path) {
     std::string js = viewer::GetJavaScript();
     callback.Run(base::RefCountedString::TakeString(&js));
     return;
   }
   content::WebContents* web_contents =
       content::WebContents::FromRenderFrameHost(
           content::RenderFrameHost::FromID(render_process_id,
                                            render_frame_id));
   DCHECK(web_contents);
-  RequestViewerHandle* request_viewer_handle =
-      new RequestViewerHandle(web_contents, scheme_, path.substr(1), callback);
+  // An empty |path| is invalid, but guard against it. If not empty, assume
+  // |path| starts with '?', which is stripped away.
+  const std::string path_after_query_separator =
+      path.size() > 0 ? path.substr(1) : "";
+  RequestViewerHandle* request_viewer_handle = new RequestViewerHandle(
+      web_contents, scheme_, path_after_query_separator, callback);
   scoped_ptr<ViewerHandle> viewer_handle = viewer::CreateViewRequest(
       dom_distiller_service_, path, request_viewer_handle);
 
   if (viewer_handle) {
     // The service returned a |ViewerHandle| and guarantees it will call
     // the |RequestViewerHandle|, so passing ownership to it, to ensure the
     // request is not cancelled. The |RequestViewerHandle| will delete itself
     // after receiving the callback.
     request_viewer_handle->TakeViewerHandle(viewer_handle.Pass());
   } else {
@@ skipping 27 matching lines @@
     const net::URLRequest* request,
     std::string* path) const {
 }
 
 std::string DomDistillerViewerSource::GetContentSecurityPolicyObjectSrc()
     const {
   return "object-src 'none'; style-src 'self';";
 }
 
 }  // namespace dom_distiller