[Offline Pages] Update Generate Page Bundle handler for testability

Update the handler for Generate Page Bundle button, so that output
text has information needed by UI automation test.

Bug: 794011
Change-Id: I67894eab2fac59cb0368e5dda53552bb889a0009
Reviewed-on: https://chromium-review.googlesource.com/823210Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524589}

chrome/browser/ui/webui/offline/offline_internals_ui_message_handler.cc

@@ -12,6 +12,7 @@
 
 #include "base/bind.h"
 #include "base/guid.h"
+#include "base/json/json_writer.h"
 #include "base/memory/ptr_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
@@ -323,8 +324,18 @@ void OfflineInternalsUIMessageHandler::HandleGeneratePageBundle(
 
   prefetch_service_->GetPrefetchDispatcher()->AddCandidatePrefetchURLs(
       offline_pages::kSuggestedArticlesNamespace, prefetch_urls);
-  ResolveJavascriptCallback(base::Value(callback_id),
-                            base::Value("Added candidate URLs."));
+  std::string message("Added candidate URLs.\n");
+  // Construct a JSON array containing all the URLs. To guard against malicious
+  // URLs that might contain special characters, we create a ListValue and then
+  // serialize it into JSON, instead of doing direct string manipulation.
+  base::ListValue urls;
+  for (const auto& prefetch_url : prefetch_urls) {
+    urls.GetList().emplace_back(prefetch_url.url.spec());
+  }
+  std::string json;
+  base::JSONWriter::Write(urls, &json);
+  message.append(json);
+  ResolveJavascriptCallback(base::Value(callback_id), base::Value(message));
 }
 
 void OfflineInternalsUIMessageHandler::HandleGetOperation(